chore(langchain): add types in agent middleware tests

[Christophe Bornet (cbornet)]

No description provided.

[Christophe Bornet (cbornet)]

I guess CustomState not inheriting from AgentState is on purpose ?

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks
⏩ 13 skipped benchmarks¹

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_{Comparing cbornet:langchain_v1-types-test-middleware (9c9edda) with master (36be77b)²}

Footnotes

1. 13 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 (18fe347) during the generation of this report, so 36be77b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩