- Add [Historical context: ...] marker pattern to stripDowngradedToolCallText
- Apply stripDowngradedToolCallText in emitBlockChunk streaming path
- Previously only stripBlockTags ran during streaming, leaking [Tool Call: ...] markers to users
- Add 7 test cases for the new pattern stripping
When replaying conversation history to Gemini, tool calls without
thought_signature are downgraded to text blocks like [Tool Call: ...].
This leaked internal technical info into user-facing chat messages.
Added stripDowngradedToolCallText filter alongside existing Minimax
filter to remove these text representations before extraction.