test: fix audit contract test isolation

[nycomp]

Summary

• Fix test isolation for audit contract tests where data persisted between test classes
• Use shared=False for fresh ServiceManager instances per test class
• Move reset_test_data() from tearDown() to tearDownClass() to reset between classes rather than per-test
• Fix trailing slash URLs to match route definitions (/traces/ not /traces)

Test Plan

Note: Contract tests (tests/contract/) are not covered by run_tests.py which only supports unit, integration, sanity, type, all categories.

Run contract tests directly:

poetry run python -m unittest discover tests/contract -v

Run all supported tests via test entrypoint:

poetry run python tests/run_tests.py all

Results

• 27 contract tests run
• "no such table: spans" errors eliminated
• Remaining failures are route implementation issues (validation, limit param type), not test isolation

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