refactor(core): eagerly validate trim_messages arguments

[Aman071106]

PR Title

refactor(core): eagerly validate trim_messages arguments

---

PR Description

This PR refactors trim_messages in langchain-core to validate its arguments (specifically token_counter, strategy, start_on, and include_system) eagerly at initialization time, rather than lazily when the returned Runnable is invoked.

---

Motivation

Previously, trim_messages was decorated with @_runnable_support, which deferred argument validation until the returned Runnable was actually invoked. This caused errors to surface late in the execution flow, making debugging harder.

This refactor removes the decorator and manually constructs a RunnableLambda, enabling fail-fast behavior by validating inputs immediately and improving developer experience.

---

Changes

• Removed the @_runnable_support decorator from trim_messages.
• Added eager validation for:
  • token_counter
  • compatibility between strategy and token_counter
  • other related arguments (start_on, include_system).
• Manually handles the messages=None case to return a RunnableLambda after validation, preserving original behavior.
• Updated test_trim_messages_bad_token_counter in test_utils.py to expect an eager ValueError.

---

Related Issues

• Addresses the TODO in libs/core/langchain_core/messages/utils.py.

---

Checklist

• I have run relevant tests locally and they pass
  (libs/core/tests/unit_tests/messages/test_utils.py)
• Added/updated tests
  • Updated test_trim_messages_bad_token_counter to assert eager validation.

[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.

Summary

✅ 13 untouched benchmarks
⏩ 21 skipped benchmarks¹

---

_{Comparing Aman071106:fix/trim-messages-eager-validation (451bef2) with master (c1f1641)²}

Footnotes

1. 21 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. ↩

2. No successful run was found on master (0438f8c) during the generation of this report, so c1f1641 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Aman071106]

Hi , just a gentle bump on this. I've rebased on the latest main to ensure there are no conflicts with the recent 100+ commits, and verified the linting passes.