fix: serialize embedded view refreshes

[vycdev2]

Summary

• Serialize per-file embedded view refreshes so filesystem and open-document events cannot update a provider concurrently.
• Keep the refresh queue usable after a rejected refresh while preserving the original error for the caller.
• Add a regression test and document the user-visible fix in the 5.3.0 changelog.

Verification

• npm install — passed; npm reported existing deprecation and vulnerability warnings.
• npm test — passed (135 tests).
• npm run compile — passed.
• npm run format:check — passed.
• git diff --check upstream/develop...HEAD — passed.
• The new test was run before implementation and failed because enqueueEmbeddedRefresh was not exported; it passed after the implementation.

Risk

• Low: only the embedded view's targeted refresh scheduling changes; full refresh behavior and provider error handling remain unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.