GitLab

Empty text blocks ({"type":"text","text":""}) cause Anthropic upstream to
return 400: "text content blocks must be non-empty". This was not caught
by the existing error detection pattern in isThinkingBlockSignatureError,
nor handled by FilterThinkingBlocksForRetry.

- Add empty text block stripping to FilterThinkingBlocksForRetry
- Fix isThinkingBlockSignatureError to match new Anthropic error format
- Add fast-path byte patterns to avoid unnecessary JSON parsing
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>