LSP: include document version when publishing diagnostics

[avifenesh]

Audit Finding (API Design, Medium)

Location: crates/agnix-lsp/src/backend.rs:191

When files are closed or skipped, the server publishes empty diagnostics with version: None. Per LSP spec, publishing without a version can cause stale diagnostics to persist in clients that cache by version.

Fix: Track document versions in the document cache and pass them to publish_diagnostics.

Found by /audit-project

[avifenesh]

Completed successfully.

PR: #544
Status: Merged to main
Commit: b5e0075

Summary

• Added document_versions: Arc<RwLock<HashMap<Url, i32>>> to Backend to track client-reported document versions
• Both documents and document_versions maps are updated under dual-lock acquisition to prevent readers from observing content and version out of sync
• All publish_diagnostics calls in validate_from_content_and_publish now pass the tracked version instead of None
• Version is always updated on did_change even when content_changes is empty, per LSP spec
• 8 new unit tests + 1 integration test added

Processed by /next-task workflow.