Recognize the server-fallback boundary marker by its signature alone
Merged
AN
7h ago+172+172+012345678901234567890123456789-108-108-012345678901234567890123456789
Recognize the server-fallback boundary marker by its signature alone. Worker delivery rebased onto main; see the commit messages.
Summary by cubic
Recognizes server-fallback boundary markers by their signature, even when a context manager clears their thinking text. This restores valid fallback boundaries on replay instead of sending invalid thinking blocks to Anthropic.
- Removes the internal marker when fallback is disabled or when requests are routed through an API key.
- Adds regression coverage for cleared and original marker text.