test: add auto-cleanup wrapper for repo fixtures #330

Summary

Add a small withRepoFixture or usingRepoFixture helper that builds a fixture, passes it to a callback, and always calls cleanup() in a finally block.

The current fixture handle exposes explicit cleanup and cleans failed builds. That is correct, but test authors still need to remember try/finally for successful builds.

Why

Canonical fixture usage should be hard to leak. As more suites migrate under #316, an auto-cleanup wrapper will keep tests concise and reduce temp-directory leaks in failure cases.

Scope

• Add a wrapper helper for callback-style fixture usage.
• Preserve direct build() access for tests that need manual lifetime control.
• Add regression tests proving cleanup runs when the callback throws.
• Document the recommended pattern in the repo fixture strategy.

Acceptance Criteria

• Tests can use a single helper call to build and clean a fixture.
• Cleanup runs on callback success and failure.
• Existing explicit cleanup behavior remains available.

Related

• test: build canonical repo fixture builder with base repos and overlays #311
• test: migrate repo-shaped suites onto the canonical repo fixture builder #316
• PR Add canonical repo fixture builder #327