fix: validate LSP documents on save

[vycdev2]

Summary

• advertise full document synchronization with save notifications so compliant LSP clients send textDocument/didSave
• validate the current open document when an editor saves it
• reuse the in-memory full-sync document text so save validation matches the latest known contents
• leave unopened documents untouched and cover both save capability advertisement and save-text lookup with focused regressions

Verification

• capability regression failed before the advertisement fix and passed afterward
• cargo test -q -p jett_lsp — passed (5 tests)
• cargo check --workspace — passed
• cargo build --workspace — passed
• cargo test -q — passed, including 432 driver fixtures
• cargo fmt --all -- --check — passed
• git diff --check — passed
• cargo clippy -q -p jett_lsp --all-targets -- -D warnings — blocked by pre-existing warnings in jett_lexer (manual_is_multiple_of, unnecessary_map_or), outside this diff

Risk

• Low: open/change synchronization retains full-document behavior; this explicitly enables and handles the standard save notification path.

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

[vycdev]

Save-triggered validation is correct in isolation and the LSP tests passed, but this branch now conflicts with current main after #88 landed. Please rebase while preserving the UTF-16 position conversions. #116 (close cleanup) and #121 (stale-version suppression) are complementary backend changes and should be integrated serially rather than overwritten; rerun the full jett_lsp suite.