tests: isolate persistence directories to prevent writes to user home

[cbagwell]

Tests were writing to ~/.openhands/agent_settings.json and other files, which pollutes the user's home directory and causes test failures when running in environments with different configurations.

Tests that did not explicitly use the mock_locations fixture would call location functions that default to ~/.openhands/. This is especially problematic when running tests via 'uv run pytest' instead of 'make test', as agents often do, since uv may use a different Python environment that does not have the same setup as the Makefile.

Add an autouse fixture in conftest.py that runs before every test and:

• Sets OPENHANDS_PERSISTENCE_DIR to an isolated temp directory
• Sets OPENHANDS_CONVERSATIONS_DIR to an isolated temp directory
• Sets PERSISTENCE_DIR (legacy) to an isolated temp directory
• Mocks os.path.expanduser('~') to return the isolated temp home

This ensures ALL tests are isolated from the user's home directory.

[all-hands-bot]

🔴 Needs improvement - Critical scope issue with test isolation

[CRITICAL ISSUES]
The autouse fixture in tests/conftest.py only applies to tests within the tests/ directory hierarchy. It does NOT apply to tui_e2e/ tests because pytest's conftest discovery is directory-scoped. Removing the HOME override from the test-binary Makefile target leaves tui_e2e/ tests completely unprotected - they will write to the user's real ~/.openhands/ directory.

[RISK ASSESSMENT]
⚠️ Risk Assessment: 🔴 HIGH

This PR introduces a critical regression: tui_e2e/ tests lose their isolation and will pollute the user's home directory, which is exactly what this PR claims to fix. The implementation is only halfway complete.

Recommendation: Create a root-level conftest.py with the autouse fixture so it applies to ALL test directories (both tests/ and tui_e2e/), or create a separate tui_e2e/conftest.py with the same fixture.

VERDICT:
❌ Needs rework - Critical scope issue must be fixed before merging

KEY INSIGHT:
Pytest's conftest.py files are directory-scoped; a fixture in tests/conftest.py does not magically apply to sibling directories like tui_e2e/.

[all-hands-bot]

🟡 Acceptable - Core implementation is sound. The autouse fixture now correctly applies to all test directories including tui_e2e/, addressing the critical scope issue from the previous review.

[RISK ASSESSMENT]
⚠️ Risk Assessment: 🟡 MEDIUM

This autouse fixture affects every test in the repository. While the implementation is sound and the previous review cycle addressed the critical scope issue, any bugs in the mocking logic could cause widespread test failures or silently allow tests to write to the real home directory. The logic is straightforward and uses standard pytest patterns, keeping risk at medium rather than high.

VERDICT:
✅ Worth merging: Core logic is sound, minor simplification suggested

KEY INSIGHT:
Moving the autouse fixture to root-level conftest.py ensures complete test isolation across all test directories, solving the critical gap where tui_e2e/ tests were previously unprotected.

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