Isolate tests in multi_process_shared with unique temp path.

[shunping]

Flaky tests:

• https://github.com/apache/beam/actions/runs/25827712861/job/75884983570
• https://github.com/apache/beam/actions/runs/25814674014/job/75839888289

These tests were failing intermittently in parallel runs because setUp was cleaning up shared files in the default temp directory, including the ones created by any on-going tests, which interferes with concurrent tests.

This fix adds an autouse pytest fixture to force all MultiProcessShared instances to use a unique tmp_path per test, and removes the unsafe global cleanup in setUp.

[gemini-code-assist]

Summary of Changes

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This pull request addresses flaky tests in the multi_process_shared module by improving test isolation. By replacing shared temporary directory usage with unique paths per test and removing problematic global cleanup routines, the changes ensure that concurrent test runs do not conflict with one another.

Highlights

• Test Isolation: Introduced an autouse pytest fixture to ensure each test case uses a unique temporary directory, preventing cross-test interference.
• Cleanup Refactoring: Removed the unsafe global cleanup logic in the setUp method that was causing intermittent failures during parallel test execution.

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[gemini-code-assist]

Code Review

This pull request introduces a pytest fixture to isolate MultiProcessShared tests by injecting a unique temporary directory path during initialization, enabling safer parallel execution. It also removes manual cleanup logic from MultiProcessSharedSpawnProcessTest. Feedback was provided regarding the fragility of the mock_init implementation, specifically concerning positional arguments and explicit None values for the path parameter, with a suggestion for a more robust function signature.

[github-actions]

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R: @damccorm for label python.

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[shunping]

PreCommit Python ML tests failed on master, so it seems irrelevant to this change.
https://github.com/apache/beam/actions/runs/25852926419