fix(core): fix trim_messages misclassification of per-message token_counter

[GAUTAM V DATLA (gautamvarmadatla)]

Fixes : #35629

I replaced the brittle annotation is BaseMessage check in trim_messages with get_type_hints() to resolve annotations to live types where possible and issubclass() to correctly match BaseMessage and its subclasses. This should fix misclassification for subclass annotations like HumanMessage, string/forward-reference annotations, and common cases involving postponed annotation evaluation. I also added a token_counter_is_per_message flag as an explicit escape hatch for lambdas and unannotated callables where auto-detection cannot work. Also added regression tests covering:

• exact BaseMessage annotation
• subclass annotation
• string annotation
• lambda with explicit override
• unannotated function with explicit override
• list-based counter backwards compatibility
• precedence of get_num_tokens_from_messages

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 13 untouched benchmarks
⏩ 27 skipped benchmarks¹

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_{Comparing gautamvarmadatla:fix/core-trim-messages-token-counter-annotation (f8a8823) with master (0f4f3f7)}

Footnotes

1. 27 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[GAUTAM V DATLA (gautamvarmadatla)]

hi All CIs seem to be failing with this error on linting. It seems to be related to yesterday's PR #35851 which introduced RUF013 violations. Could you please have a look?

I tried merging my PRs #35420, #35239, and #35630, but all three failed

cc ccurme (@ccurme) , Mason Daugherty (@mdrxy)

Error: langchain_text_splitters/base.py:230:26: RUF013 PEP 484 prohibits implicit `Optional`
  help: Convert to `T | None`
Error: langchain_text_splitters/base.py:307:26: RUF013 PEP 484 prohibits implicit `Optional`
  help: Convert to `T | None`
make: *** [Makefile:46: lint_package] Error 1
Error: Process completed with exit code 2.