feat(lsp): implement real-time diagnostics and decouple core

[sonesuke]

Summary

This PR implements real-time diagnostics updates for the LSP server and refactors the core module to remove dependencies on LSP types.

Changes

• LSP Backend:
  • Implemented in-memory document synchronization to reflect changes immediately without saving.
  • Added logic to publish diagnostics for all workspace files to ensuring stale errors on closed files are cleared.
  • Implemented path canonicalization for reliable document overrides lookup.
• Core Module:
  • Refactored check_workspace and collect_workspace_all to accept HashMap<PathBuf, String> for overrides, decoupling core from tower_lsp and dashmap.
  • Added path canonicalization handling in override lookups.
• CLI:
  • Updated handlers to pass None for the new overrides argument in collect_workspace_all.

Verification

• Verified manually in VS Code that diagnostics update in real-time and cross-file reference errors clear immediately upon fixing the definition.
• Verified cargo check, cargo build, cargo fmt, and cargo clippy pass.