fix(executor/xai): drop orphaned tool_choice when Claude tools array is empty

When Claude Code sends a stop-hook evaluator request (or any request
without tools), the payload includes "tools": [] (empty array). The
claude->codex translator unconditionally emits tools: [] + tool_choice:
"auto" + parallel_tool_calls: true into the Codex Responses shape.

When that payload is routed to xAI, the upstream rejects with HTTP 400:
"A tool_choice was set on the request but no tools were specified."

Fix entirely in the xAI executor (translator package is policy-locked):
add normalizeXAIToolChoiceForTools() after normalizeXAITools() to drop
tool_choice and parallel_tool_calls whenever tools end up absent or
empty (covering both the empty-from-source case and the
all-filtered-out case where every tool was an unsupported type such as
tool_search or image_generation).

Per code-review feedback: always remove parallel_tool_calls when tools
are missing (not gated on tool_choice presence) and existence-check
each key before sjson delete to avoid unnecessary JSON parse/copy.

Verification:
- go build -o test-output ./cmd/server
- go test ./internal/runtime/executor/... -count=1
- 5 new regression tests cover empty / missing / present / orphaned
  parallel_tool_calls / no-op-when-both-absent.

internal/runtime/executor/xai_executor.go

@@ -506,6 +506,7 @@ func (e *XAIExecutor) prepareResponsesRequest(ctx context.Context, req cliproxye
 	body, _ = sjson.DeleteBytes(body, "safety_identifier")
 	body, _ = sjson.DeleteBytes(body, "stream_options")
 	body = normalizeXAITools(body)
+	body = normalizeXAIToolChoiceForTools(body)
 	body = normalizeXAIInputReasoningItems(body)
 	body = normalizeCodexInstructions(body)
 	body = sanitizeXAIResponsesBody(body, baseModel)
@@ -715,6 +716,28 @@ func normalizeXAITools(body []byte) []byte {
 	return updated
 }
 
+// normalizeXAIToolChoiceForTools drops tool_choice and parallel_tool_calls
+// when tools are absent or empty (including after normalizeXAITools filtering).
+// xAI rejects payloads that include tool_choice without any tools defined.
+// Existence checks avoid unnecessary sjson parse/copy passes.
+func normalizeXAIToolChoiceForTools(body []byte) []byte {
+	tools := gjson.GetBytes(body, "tools")
+	hasTools := tools.Exists() && tools.IsArray() && len(tools.Array()) > 0
+	if hasTools {
+		return body
+	}
+	if tools.Exists() {
+		body, _ = sjson.DeleteBytes(body, "tools")
+	}
+	if gjson.GetBytes(body, "tool_choice").Exists() {
+		body, _ = sjson.DeleteBytes(body, "tool_choice")
+	}
+	if gjson.GetBytes(body, "parallel_tool_calls").Exists() {
+		body, _ = sjson.DeleteBytes(body, "parallel_tool_calls")
+	}
+	return body
+}
+
 func normalizeXAITool(tool gjson.Result) ([]byte, bool, bool) {
 	toolType := tool.Get("type").String()
 	changed := false