Fix stale rejected custom editor model cache on reopen

[murataslan1]

Fixes #268301.

Summary

• clear cached model entries when model creation promise rejects in CustomEditorModelManager.add
• allow subsequent open attempts to recreate custom editor models after transient failures (for example missing file at first open)
• add a regression test covering reject-then-recover behavior

Test Plan

• npx tsx ./node_modules/mocha/bin/mocha --ui tdd src/vs/workbench/contrib/customEditor/test/common/customEditorModelManager.test.ts --timeout 10000

[Copilot]

Pull request overview

This PR fixes issue #268301 where custom editors would cache rejected model creation promises and never retry, even after the underlying issue (e.g., missing file) was resolved.

Changes:

• Modified CustomEditorModelManager.add() to clear cached model entries when model creation promises reject
• Added a regression test verifying that rejected models can be recreated on subsequent open attempts

Reviewed changes

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

File	Description
src/vs/workbench/contrib/customEditor/common/customEditorModelManager.ts	Adds error handling to clear rejected model entries from the cache
src/vs/workbench/contrib/customEditor/test/common/customEditorModelManager.test.ts	Adds a new test suite with regression test for reject-then-recover behavior

[Copilot]

The test logic is inconsistent. Lines 62-64 check if retained exists and then assert it rejects, but line 65 then asserts that retained is undefined. If retained is undefined (line 65), then lines 62-64 will never execute. The assertion on line 65 should come first, or the entire if block should be removed since we expect retained to be undefined after the model is cleared from the cache.

Suggested change

	if (retained) {
	await assert.rejects(retained, error);
	}

[murataslan1]

Rebased/refreshed this PR and force-pushed the branch to current main.

Still contains the same intended fix:

• clear cached custom editor model entries when model creation rejects
• add a regression test that verifies reject-then-recover behavior

Please re-review when convenient.

[murataslan1]

Friendly follow-up — this PR fixes stale rejected custom editor model cache on reopen. Would appreciate a review when available.