fix(core): treat empty tool chunk ids as missing in merge

[Guofang.Tang (T1mn)]

Fixes #35395

Tool call chunk aggregation treated id="" as a conflicting identifier, which prevented chunks with the same index from merging and could break streamed tool-argument reconstruction. This change treats empty ids as missing ids during list merge so chunks continue to merge correctly.

How did you verify your code works?

• uv run --project libs/core --group test pytest libs/core/tests/unit_tests/test_messages.py::test_message_chunks
• uv run --project libs/core --group test pytest libs/core/tests/unit_tests/test_messages.py

Breaking changes

• None

Depends on

• None

[codspeed-hq]

Merging this PR will degrade performance by 22.46%

⚠️ 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 regressed benchmarks
⏩ 23 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	test_import_time[CallbackManager]	286.4 ms	325.5 ms	-12.01%
❌	WallTime	test_import_time[LangChainTracer]	410.6 ms	464.9 ms	-11.67%
❌	WallTime	test_import_time[InMemoryRateLimiter]	158.6 ms	178.1 ms	-10.95%
❌	WallTime	test_import_time[InMemoryVectorStore]	552.5 ms	623.6 ms	-11.4%
❌	WallTime	test_import_time[Runnable]	444.5 ms	504.4 ms	-11.87%
❌	WallTime	test_import_time[HumanMessage]	239.3 ms	269.5 ms	-11.19%
❌	WallTime	test_import_time[ChatPromptTemplate]	549.8 ms	648.3 ms	-15.2%
❌	WallTime	test_import_time[tool]	473.5 ms	559.6 ms	-15.39%
❌	WallTime	test_import_time[Document]	172.1 ms	192.3 ms	-10.47%
❌	WallTime	test_import_time[PydanticOutputParser]	477.6 ms	553.7 ms	-13.74%
❌	WallTime	test_async_callbacks_in_sync	18.4 ms	23.7 ms	-22.46%
❌	WallTime	test_import_time[RunnableLambda]	442 ms	501.5 ms	-11.86%
❌	WallTime	test_import_time[BaseChatModel]	482.7 ms	547.5 ms	-11.83%

---

_{Comparing T1mn:fix/core-tool-call-chunk-empty-id-merge (d5eb8ff) with master (4ffb584)}

Footnotes

1. 23 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. ↩