fix: resolve default view for markdown files on first startup

[maruthang]

Fixes #244597

Summary

Bug: Default view for markdown files doesn't work on first startup -- the user-configured custom editor (e.g., markdown preview) is not used when opening .md files for the first time.

Root Cause: The EditorResolverService uses a cache of glob patterns (persisted via StorageService) to decide whether to wait for extensions to register before resolving editors. On first startup, this cache is empty because it is only populated during onWillSaveState, so the resolver skips waiting for extensions and falls back to the default text editor.

Fix: Added a resourceMatchesUserAssociation() method that checks if the resource matches any user-configured editorAssociations pointing to a non-default editor. This is used as a fallback alongside the existing cache check, ensuring the resolver waits for extensions even on first startup when the cache is empty.

Changes

• src/vs/workbench/services/editor/browser/editorResolverService.ts: Added resourceMatchesUserAssociation() method and updated the cache check condition to also consider user-configured editor associations as a fallback.
• src/vs/workbench/services/editor/test/browser/editorResolverService.test.ts: Added regression test that verifies user-configured editor associations resolve correctly even with an empty cache (simulating first startup).

Test Plan

• Added regression test User-configured editor association resolves on first startup with empty cache #244597
• Verify on a fresh VS Code profile that opening a .md file uses the configured custom editor (e.g., markdown preview) on first startup
• Verify existing editor resolution behavior is unchanged for subsequent startups (cache populated)
• Verify files without user associations still resolve to the default editor

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@bpasero

Matched files:

• src/vs/workbench/services/editor/browser/editorResolverService.ts
• src/vs/workbench/services/editor/test/browser/editorResolverService.test.ts

[mjbvz]

thanks