feat(antigravity): configurable signature cache with bypass-mode validation

Antigravity 的 Claude thinking signature 处理新增 cache/bypass 双模式，
并为 bypass 模式实现按 SIGNATURE-CHANNEL-SPEC.md 的签名校验。

新增 antigravity-signature-cache-enabled 配置项（默认 true）：
- cache mode（true）：使用服务端缓存的签名，行为与原有逻辑完全一致
- bypass mode（false）：直接使用客户端提供的签名，经过校验和归一化

支持配置热重载，运行时可切换模式。

校验流程：
1. 剥离历史 cache-mode 的 'modelGroup#' 前缀（如 claude#Exxxx → Exxxx）
2. 首字符必须为 'E'（单层编码）或 'R'（双层编码），否则拒绝
3. R 开头：base64 解码 → 内层必须以 'E' 开头 → 继续单层校验
4. E 开头：base64 解码 → 首字节必须为 0x12（Claude protobuf 标识）
5. 所有合法签名归一化为 R 形式（双层 base64）发往 Antigravity 后端

非法签名处理策略：
- 非严格模式（默认）：translator 静默丢弃无签名的 thinking block
- 严格模式（antigravity-signature-bypass-strict: true）：
  executor 层在请求发往上游前直接返回 HTTP 400

按 SIGNATURE-CHANNEL-SPEC.md 解析 Claude 签名的完整 protobuf 结构：
- Top-level Field 2（容器）→ Field 1（渠道块）
- 渠道块提取：channel_id (Field 1)、infrastructure (Field 2)、
  model_text (Field 6)、field7 (Field 7)
- 计算 routing_class、infrastructure_class、schema_features
- 使用 google.golang.org/protobuf/encoding/protowire 解析

- resolveThinkingSignature 拆分为 resolveCacheModeSignature / resolveBypassModeSignature
- hasResolvedThinkingSignature：mode-aware 签名有效性判断
  （cache: len>=50 via HasValidSignature，bypass: non-empty）
- validateAntigravityRequestSignatures：executor 预检，
  仅在 bypass + strict 模式下拦截非法签名返回 400
- 响应侧签名缓存逻辑与 cache mode 集成
- Cache mode 行为完全保留：无 '#' 前缀的原生签名静默丢弃

internal/runtime/executor/antigravity_executor.go

@@ -23,10 +23,12 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/cache"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/misc"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/runtime/executor/helps"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/thinking"
+	antigravityclaude "github.com/router-for-me/CLIProxyAPI/v6/internal/translator/antigravity/claude"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/util"
 	sdkAuth "github.com/router-for-me/CLIProxyAPI/v6/sdk/auth"
 	cliproxyauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
@@ -158,6 +160,24 @@ func newAntigravityHTTPClient(ctx context.Context, cfg *config.Config, auth *cli
 	return client
 }
 
+func validateAntigravityRequestSignatures(from sdktranslator.Format, rawJSON []byte) error {
+	if from.String() != "claude" {
+		return nil
+	}
+	if cache.SignatureCacheEnabled() {
+		return nil
+	}
+	if !cache.SignatureBypassStrictMode() {
+		// Non-strict bypass: let the translator handle invalid signatures
+		// by dropping unsigned thinking blocks silently (no 400).
+		return nil
+	}
+	if err := antigravityclaude.ValidateClaudeBypassSignatures(rawJSON); err != nil {
+		return statusErr{code: http.StatusBadRequest, msg: err.Error()}
+	}
+	return nil
+}
+
 // Identifier returns the executor identifier.
 func (e *AntigravityExecutor) Identifier() string { return antigravityAuthType }
 
@@ -479,14 +499,6 @@ func (e *AntigravityExecutor) Execute(ctx context.Context, auth *cliproxyauth.Au
 		return e.executeClaudeNonStream(ctx, auth, req, opts)
 	}
 
-	token, updatedAuth, errToken := e.ensureAccessToken(ctx, auth)
-	if errToken != nil {
-		return resp, errToken
-	}
-	if updatedAuth != nil {
-		auth = updatedAuth
-	}
-
 	reporter := helps.NewUsageReporter(ctx, e.Identifier(), baseModel, auth)
 	defer reporter.TrackFailure(ctx, &err)
 
@@ -498,6 +510,16 @@ func (e *AntigravityExecutor) Execute(ctx context.Context, auth *cliproxyauth.Au
 		originalPayloadSource = opts.OriginalRequest
 	}
 	originalPayload := originalPayloadSource
+	if errValidate := validateAntigravityRequestSignatures(from, originalPayload); errValidate != nil {
+		return resp, errValidate
+	}
+	token, updatedAuth, errToken := e.ensureAccessToken(ctx, auth)
+	if errToken != nil {
+		return resp, errToken
+	}
+	if updatedAuth != nil {
+		auth = updatedAuth
+	}
 	originalTranslated := sdktranslator.TranslateRequest(from, to, baseModel, originalPayload, false)
 	translated := sdktranslator.TranslateRequest(from, to, baseModel, req.Payload, false)
 
@@ -655,14 +677,6 @@ attemptLoop:
 func (e *AntigravityExecutor) executeClaudeNonStream(ctx context.Context, auth *cliproxyauth.Auth, req cliproxyexecutor.Request, opts cliproxyexecutor.Options) (resp cliproxyexecutor.Response, err error) {
 	baseModel := thinking.ParseSuffix(req.Model).ModelName
 
-	token, updatedAuth, errToken := e.ensureAccessToken(ctx, auth)
-	if errToken != nil {
-		return resp, errToken
-	}
-	if updatedAuth != nil {
-		auth = updatedAuth
-	}
-
 	reporter := helps.NewUsageReporter(ctx, e.Identifier(), baseModel, auth)
 	defer reporter.TrackFailure(ctx, &err)
 
@@ -674,6 +688,16 @@ func (e *AntigravityExecutor) executeClaudeNonStream(ctx context.Context, auth *
 		originalPayloadSource = opts.OriginalRequest
 	}
 	originalPayload := originalPayloadSource
+	if errValidate := validateAntigravityRequestSignatures(from, originalPayload); errValidate != nil {
+		return resp, errValidate
+	}
+	token, updatedAuth, errToken := e.ensureAccessToken(ctx, auth)
+	if errToken != nil {
+		return resp, errToken
+	}
+	if updatedAuth != nil {
+		auth = updatedAuth
+	}
 	originalTranslated := sdktranslator.TranslateRequest(from, to, baseModel, originalPayload, true)
 	translated := sdktranslator.TranslateRequest(from, to, baseModel, req.Payload, true)
 
@@ -1080,14 +1104,6 @@ func (e *AntigravityExecutor) ExecuteStream(ctx context.Context, auth *cliproxya
 
 	ctx = context.WithValue(ctx, "alt", "")
 
-	token, updatedAuth, errToken := e.ensureAccessToken(ctx, auth)
-	if errToken != nil {
-		return nil, errToken
-	}
-	if updatedAuth != nil {
-		auth = updatedAuth
-	}
-
 	reporter := helps.NewUsageReporter(ctx, e.Identifier(), baseModel, auth)
 	defer reporter.TrackFailure(ctx, &err)
 
@@ -1099,6 +1115,16 @@ func (e *AntigravityExecutor) ExecuteStream(ctx context.Context, auth *cliproxya
 		originalPayloadSource = opts.OriginalRequest
 	}
 	originalPayload := originalPayloadSource
+	if errValidate := validateAntigravityRequestSignatures(from, originalPayload); errValidate != nil {
+		return nil, errValidate
+	}
+	token, updatedAuth, errToken := e.ensureAccessToken(ctx, auth)
+	if errToken != nil {
+		return nil, errToken
+	}
+	if updatedAuth != nil {
+		auth = updatedAuth
+	}
 	originalTranslated := sdktranslator.TranslateRequest(from, to, baseModel, originalPayload, true)
 	translated := sdktranslator.TranslateRequest(from, to, baseModel, req.Payload, true)
 
@@ -1307,6 +1333,16 @@ func (e *AntigravityExecutor) Refresh(ctx context.Context, auth *cliproxyauth.Au
 func (e *AntigravityExecutor) CountTokens(ctx context.Context, auth *cliproxyauth.Auth, req cliproxyexecutor.Request, opts cliproxyexecutor.Options) (cliproxyexecutor.Response, error) {
 	baseModel := thinking.ParseSuffix(req.Model).ModelName
 
+	from := opts.SourceFormat
+	to := sdktranslator.FromString("antigravity")
+	respCtx := context.WithValue(ctx, "alt", opts.Alt)
+	originalPayloadSource := req.Payload
+	if len(opts.OriginalRequest) > 0 {
+		originalPayloadSource = opts.OriginalRequest
+	}
+	if errValidate := validateAntigravityRequestSignatures(from, originalPayloadSource); errValidate != nil {
+		return cliproxyexecutor.Response{}, errValidate
+	}
 	token, updatedAuth, errToken := e.ensureAccessToken(ctx, auth)
 	if errToken != nil {
 		return cliproxyexecutor.Response{}, errToken
@@ -1318,10 +1354,6 @@ func (e *AntigravityExecutor) CountTokens(ctx context.Context, auth *cliproxyaut
 		return cliproxyexecutor.Response{}, statusErr{code: http.StatusUnauthorized, msg: "missing access token"}
 	}
 
-	from := opts.SourceFormat
-	to := sdktranslator.FromString("antigravity")
-	respCtx := context.WithValue(ctx, "alt", opts.Alt)
-
 	// Prepare payload once (doesn't depend on baseURL)
 	payload := sdktranslator.TranslateRequest(from, to, baseModel, req.Payload, false)