fix(mock): do not persist snapshots on close in playback mode

[GeoffreyBooth]

Problem

SnapshotAgent.close() unconditionally delegates to the recorder's close(), which saves snapshots to disk whenever any are held — with no regard for the current mode:

// lib/mock/snapshot-recorder.js
async close () {
  if (this.#snapshotPath && this.#snapshots.size !== 0) {
    await this.saveSnapshots()
  }
  this.destroy()
}

In playback mode this is a problem because findSnapshot() mutates each matched snapshot on every call:

// lib/mock/snapshot-recorder.js — findSnapshot()
const currentCallCount = snapshot.callCount || 0
const responseIndex = Math.min(currentCallCount, snapshot.responses.length - 1)
snapshot.callCount = currentCallCount + 1

And callCount is serialized by saveSnapshots(). So the chain is:

playback run → every matched request bumps callCount in memory → close() flushes unconditionally → the snapshot file is rewritten on disk.

The result is spurious diffs in committed snapshot fixtures after a plain playback run (no recording involved). For projects that store fixtures in git (e.g. via Git LFS), every test run produces a new file hash and noisy, meaningless diffs. The persisted callCount is also stale state — loadSnapshots() does not reset it — so it has no business being written back during playback.

Fix

Gate persistence at SnapshotAgent.close(), which is the only layer that knows the mode. Record/update modes still save via recorder.close(); playback only cleans up timers via recorder.destroy():

async close () {
  if (this[kSnapshotMode] === 'playback') {
    this[kSnapshotRecorder].destroy()
  } else {
    await this[kSnapshotRecorder].close()
  }
  await this[kRealAgent]?.close()
  await super.close()
}

The public saveSnapshots() is intentionally left untouched, so an explicit save still works in any mode — only the automatic save-on-close is suppressed during playback.

Test

Added a regression test in the existing Close Method block (test/snapshot-testing.js): record a fixture, capture its exact on-disk bytes, run a playback session with several matched requests (mutating callCount in memory), close(), and assert the file is byte-for-byte unchanged. It fails on main and passes with this change.

Note / possible follow-up

The opt-in auto-flush timer (autoFlush: true) also calls saveSnapshots() and is likewise not mode-gated, so it could still rewrite a fixture during a long-running playback session. That's a separate, opt-in path (default off) and out of scope here, but worth flagging — happy to address it in a follow-up if maintainers prefer.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.23%. Comparing base (9b1d58f) to head (7ba095c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5359   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files         110      110           
  Lines       36668    36676    +8     
=======================================
+ Hits        34187    34196    +9     
+ Misses       2481     2480    -1     

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[mcollina]

lgtm