Fix MockPlugin clobbering the active mock when a non-active duplicate is removed

[durvesh1992]

Summary

When a manual-mock file with duplicates is removed, MockPlugin#onFileRemoved unconditionally reassigns the active mock to an arbitrary remaining duplicate:

duplicates.delete(posixRelativePath);
if (duplicates.size === 1) {
  this.#raw.duplicates.delete(mockName);
}
// Set the mock to a remaining duplicate. Should never be empty.
const remaining = nullthrows(duplicates.values().next().value);
this.#raw.mocks.set(mockName, remaining);   // always overwrites the active mock

It never checks whether the removed file was the active mock (this.#raw.mocks.get(mockName)). When a non-active duplicate is removed and 3+ duplicates exist, values().next().value returns the first-inserted path, silently changing which mock resolves — even though the previously-active mock still exists on disk.

The existing 2-duplicate "recovers from duplicates" test masks this because the single remaining element is coincidentally always the right answer.

Fix

Capture whether the removed file was the active mock before deleting, and only reassign in that case.

Test plan

jest packages/metro-file-map/src/plugins/mocks/__tests__/

Added a regression test: 3 duplicates (a, b, c → active c), remove the non-active b, assert the active mock stays c. Fails before (becomes a), passes after. Full MockPlugin + getMockName suites: 13 pass, no regressions.