docs(langchain): correct import paths in agent middleware docstrings

[Kanav Bansal (bansalkanav)]

Summary

Updates the example import paths in ModelFallbackMiddleware and TodoListMiddleware docstrings to use the correct module path. The previous imports referenced non-existent modules, which could cause confusion for users following the documentation.

Impact

Improves documentation accuracy and developer experience. No functional changes.

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks
⏩ 13 skipped benchmarks¹

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_{Comparing bansalkanav:master (a6449c7) with master (7b09eb7)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 (3dd0ad9) during the generation of this report, so 7b09eb7 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩