unit and integration tests for filesystem pause/resume integrity

[matthewlouisbrockman]

Add unit and integration coverage for zero-copy-fs, covering Cache.ExportToDiff semantics and filesystem integrity across pause/resume for contiguous, scattered, and zeroed/truncate write patterns.

includes an opt-in stress test via env TESTS_FS_INTEGRITY_STRESS=1 for several gigabyte pause/resume validation (not automatically run on ci to avoid taking forever)

[cursor]

PR Summary

Low Risk
Test-only changes; main risk is increased test runtime/flakiness due to new integration and optional stress coverage around pause/resume flows.

Overview
Adds new unit and integration tests to validate zero-copy filesystem snapshot behavior: Cache.ExportToDiff now has explicit coverage ensuring zero-filled dirty blocks are still exported as dirty payloads (and mapped to the snapshot build) and that non-contiguous dirty ranges preserve export ordering. Adds an integration suite that writes files with contiguous/scattered patterns plus zeroing+truncate, then repeatedly pause/resumes sandboxes and asserts file hash/size stability, with an opt-in large stress variant gated by TESTS_FS_INTEGRITY_STRESS.

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[claude]

LGTM — test-only PR with three minor nits already flagged inline.

Extended reasoning...

Overview

This PR adds unit tests to cache_test.go covering Cache.ExportToDiff semantics (zero dirty blocks, mixed dirty blocks, non-contiguous ranges) and a new integration test file for filesystem pause/resume integrity across contiguous, scattered, and zeroed/truncate write patterns.

Security risks

None. The changes are entirely test code with no modifications to production logic.

Level of scrutiny

Low. Test-only additions that exercise already-merged production code. The unit tests follow established patterns; the integration tests are gated behind the existing test infrastructure and an opt-in env var for the stress variant.

Other factors

Three nits were flagged inline — a missing resp.JSON201 nil check, a hardcoded path string that diverges from a Go constant, and a no-op for i := 0; i < 1 loop. All are in test code and none affect correctness of the tests under normal conditions. The overall coverage added is valuable and the structure is sound.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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