fix: strip _claudeCodeRequiresLowercaseToolNames before serialization

The sentinel field set by remapToolNamesInRequest() was being included in
the JSON body sent to Anthropic, which rejects unknown top-level fields
with 400 invalid_request_error. Stripped before JSON.stringify in both
code paths:
- buildAndSignClaudeCodeRequest (CC bridge / anthropic-compatible-cc-*)
- base executor serialization path (native claude/ provider)

Pre-existing bug, not introduced by issue #2260 changes.

open-sse/executors/base.ts

@@ -856,6 +856,11 @@ export class BaseExecutor {
         // CLI fingerprint ordering — always-on for native Claude OAuth, opt-in
         // for other providers. Header + body field order is itself a fingerprint.
         let finalHeaders = headers;
+        // Strip internal sentinel fields set by remapToolNamesInRequest before
+        // serializing — Anthropic rejects unknown top-level fields (issue #2260).
+        delete (transformedBody as Record<string, unknown>)[
+          "_claudeCodeRequiresLowercaseToolNames"
+        ];
         let bodyString = JSON.stringify(transformedBody);
 
         const shouldFingerprint =

open-sse/services/claudeCodeCompatible.ts

@@ -354,7 +354,8 @@ export async function buildAndSignClaudeCodeRequest(
     obfuscateInBody(body);
   }
 
-  // Step 7: Serialize with CCH placeholder
+  // Step 7: Serialize with CCH placeholder (strip internal sentinel fields)
+  delete (body as Record<string, unknown>)["_claudeCodeRequiresLowercaseToolNames"];
   const serialized = JSON.stringify(body);
 
   // Step 8: Sign with xxHash64