fix(langchain): sanitize anthropic cache markers on fallback retries

[Hunter Lovell (hntrl)]

Summary

Fixes #33709.

When AnthropicPromptCachingMiddleware runs before ModelFallbackMiddleware, Anthropic-specific cache_control markers can leak into fallback attempts targeting non-Anthropic models. This change makes fallback retries sanitize those markers only on non-primary attempts, preserving primary-call behavior and avoiding API changes.

Related: langchain-ai/deepagentsjs#551.

Changes

libs/langchain_v1

• Added a private fallback sanitizer in model_fallback.py to remove Anthropic cache_control markers from fallback requests.
• Sanitization covers model_settings, system/content message blocks, and tool payloads (BaseTool.extras plus dict-style tools/extras).
• Applied sanitization in both sync and async fallback retry paths (wrap_model_call and awrap_model_call) while leaving the primary attempt unchanged.

libs/langchain_v1 tests

• Added sync and async regression tests in test_model_fallback.py that simulate primary failure and assert fallback calls only succeed when cache markers are stripped.
• Verified non-cache settings (for example temperature and top_p) are preserved on fallback.
• Preserved existing fallback behavior coverage (ordering, exhaustion, and error propagation).