* Fix: Map Gemini cached_tokens to Langfuse cache_read_input_tokens
Fixes#18520
## Problem
Langfuse integration was not capturing cached tokens from Gemini models.
Gemini returns cached tokens in `usage.prompt_tokens_details.cached_tokens`,
but Langfuse only read from top-level `usage.cache_read_input_tokens`
(which only Anthropic populates).
## Solution
Updated langfuse.py to check both locations:
1. First check top-level cache_read_input_tokens (for Anthropic)
2. Then check prompt_tokens_details.cached_tokens (for Gemini, OpenAI, others)
This ensures all providers' cached tokens are properly reported to Langfuse.
## Changes
- Modified litellm/integrations/langfuse/langfuse.py (lines 742-761)
- Added 3 unit tests in tests/test_litellm/integrations/langfuse/test_gemini_cached_tokens.py
- All existing Langfuse tests still pass (11/11)
## Testing
- test_cached_tokens_extraction: Verifies Gemini cached_tokens extraction
- test_cached_tokens_not_present: Backward compatibility (no cached_tokens)
- test_cached_tokens_is_zero: Edge case when cached_tokens = 0
* Refactor: Extract cache token logic into helper function
Address review feedback from @officer47p
- Created _extract_cache_read_input_tokens() helper function
- Reduces code bloat in _log_langfuse_v2 method
- Improves testability and reusability
- All tests still passing (11/11)
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