Fix BidirectionalMap leaving a stale reverse entry on key update

[dsavy4]

BidirectionalMap.set updates the forward map (_m1) and the reverse map (_m2), but when an existing key is reassigned it never removes the reverse entry for the key's previous value. That leaves a stale reverse mapping.

Repro:

const map = new BidirectionalMap();
map.set('a', 1);
map.set('a', 2);
map.get('a');    // 2
map.getKey(2);   // 'a'
map.getKey(1);   // 'a'  <- stale, should be undefined

getKey(1) still resolves to 'a' even though 'a' now maps to 2. This can surface wherever a key's value is updated, for example the marker decorations service (IMarker to decoration id) when a marker's decoration id changes.

The fix deletes the previous value's reverse entry when an existing key is reassigned. True one-to-one behavior is preserved, and a regression test covering key updates is added.

[Copilot]

Pull request overview

Fixes stale reverse mappings when a BidirectionalMap key is reassigned.

Changes:

• Removes the previous reverse entry before updating a key.
• Adds regression coverage for key reassignment.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/vs/base/common/map.ts	Cleans up the old reverse mapping during updates.
src/vs/base/test/common/map.test.ts	Tests forward and reverse lookups after reassignment.