perf(core): defer tracer imports in runnables/base.py to call time

[Sydney Runkle (sydney-runkle)]

Summary

• Removes 4 eager top-level tracer imports from runnables/base.py (event_stream, log_stream, root_listeners, _streaming) that were pulled in on every from langchain_core.language_models import BaseChatModel
• Moves them to TYPE_CHECKING and adds deferred imports at the call sites (astream_log, astream_events, with_listeners, with_alisteners)
• Keeps _StreamingCallbackHandler at top level since it's a lightweight module needed for isinstance checks

Motivation

The tracer import chain was the dominant cost in from langchain_core.language_models import BaseChatModel:

runnables/base.py
  → tracers.event_stream → tracers.log_stream → tracers.base
  → tracers.schemas → langsmith.run_trees → langsmith.schemas (~32ms self)

This eliminates ~76ms from the import chain, bringing BaseChatModel import from ~291ms down to ~160ms.

Test plan

• cd libs/core && uv run --group test pytest tests/unit_tests/runnables/ -x -q

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will improve performance by 52.38%

⚡ 7 improved benchmarks
✅ 8 untouched benchmarks
🆕 7 new benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_import_time[tool]	1,406 ms	959.6 ms	+46.52%
⚡	WallTime	test_import_time[InMemoryVectorStore]	1.5 s	1 s	+43.94%
⚡	WallTime	test_import_time[BaseChatModel]	1,373.2 ms	962.6 ms	+42.65%
⚡	WallTime	test_import_time[ChatPromptTemplate]	1.6 s	1.1 s	+52.38%
⚡	WallTime	test_import_time[Runnable]	1,270.7 ms	857.3 ms	+48.22%
⚡	WallTime	test_import_time[PydanticOutputParser]	1,393.9 ms	953.3 ms	+46.23%
⚡	WallTime	test_import_time[RunnableLambda]	1,268.2 ms	857.1 ms	+47.96%
🆕	Simulation	test_create_agent_medium_tools	N/A	10.6 ms	N/A
🆕	Simulation	test_tool_call_schema_repeated_access	N/A	20.1 µs	N/A
🆕	Simulation	test_tool_args_repeated_access	N/A	19.9 µs	N/A
🆕	Simulation	test_create_agent_small_tools	N/A	9.6 ms	N/A
🆕	Simulation	test_create_agent_large_tools_with_middleware	N/A	19.3 ms	N/A
🆕	Simulation	test_create_agent_large_tools_memory	N/A	81.3 ms	N/A
🆕	Simulation	test_create_agent_large_tools	N/A	13.5 ms	N/A

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_{Comparing sr/deferred-imports (0545fee) with master (fb6ab99)¹}

Footnotes

1. No successful run was found on master (acc5498) during the generation of this report, so fb6ab99 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩