db: harden Postgres fixture cleanup

[sputn1ck]

Summary

• tear down a newly created Postgres fixture if opening the first store fails
• evict path-cache entries before purging the fixture container during cleanup
• document the duplicate-creator branch as a defensive invariant guard

Review comment analysis

• Copilot was right: after NewTestPgFixture, a NewPostgresStore failure could skip TearDown and leak the fixture semaphore slot.
• Gemini was directionally right: cleanup should delete the cache entry before Docker purge so no caller can observe a tearing-down fixture. This is defensive under the documented per-test path invariant, but cheap and safer.
• Claude's nit about the double-check branch being defensive-only was also valid, so this adds a short comment.

Verification

• go test -tags="dev test_postgres nolog" ./db -run '^$'
• make fmt-changed
• make lint-changed-local

Follow-up to #864.

[gemini-code-assist]

Code Review

This pull request modifies db/test_postgres.go to improve resource cleanup and error handling during test database setup. Specifically, it ensures that sqlFixture.TearDown is called if NewPostgresStore fails, adds a defensive comment regarding sequential dbPath reopening, and reorders the operations in t.Cleanup so that the fixture is removed from the global map before being torn down. There are no review comments, so I have no feedback to provide.

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[Copilot]

Pull request overview

This PR hardens the lifecycle of Postgres test fixtures in db to prevent resource/semaphore leaks and reduce the chance that any code observes a fixture while it is tearing down, building on the fixture-cache changes introduced in #864.

Changes:

• Ensure a newly created Postgres fixture is torn down if the initial NewPostgresStore open/migration step fails.
• Evict dbPath cache entries before purging the fixture container during t.Cleanup.
• Add a short comment clarifying the “duplicate creator” branch as a defensive invariant guard.

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