[antauth-alfonso]

Recognize the server-fallback boundary marker by its signature alone. Worker delivery rebased onto main; see the commit messages.

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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.

^{Written for commit 74745ac. Summary will update on new commits.}