Keep Claude Code compatibility work low-risk and reviewable

This change stops short of broader Claude Code runtime alignment and instead
hardens two safe edges: builtin tool prefix handling and source-informed
sentinel coverage for future drift checks.

Constraint: Must preserve existing default behavior for current users
Rejected: Implement control-plane/session alignment now | too much runtime risk for a first slice
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Treat the new fixtures as compatibility sentinels, not a full Claude Code schema contract
Tested: go test ./test/...; go test ./sdk/translator/...; go test ./internal/runtime/executor -run 'Claude|Builtin|Tool'; go test ./...
Not-tested: End-to-end Claude Code direct-connect/session runtime behavior

internal/runtime/executor/claude_executor.go

@@ -919,12 +919,9 @@ func applyClaudeToolPrefix(body []byte, prefix string) []byte {
 		return body
 	}
 
-	// Collect built-in tool names (those with a non-empty "type" field) so we can
-	// skip them consistently in both tools and message history.
-	builtinTools := map[string]bool{}
-	for _, name := range []string{"web_search", "code_execution", "text_editor", "computer"} {
-		builtinTools[name] = true
-	}
+	// Collect built-in tool names from the authoritative fallback seed list and
+	// augment it with any typed built-ins present in the current request body.
+	builtinTools := augmentClaudeBuiltinToolRegistry(body, nil)
 
 	if tools := gjson.GetBytes(body, "tools"); tools.Exists() && tools.IsArray() {
 		tools.ForEach(func(index, tool gjson.Result) bool {