fix: clear stale LSP diagnostics on document close

[vycdev2]

Summary

• Clear published diagnostics when an LSP document is closed so editors do not retain stale errors.
• Extract diagnostic-to-LSP mapping into a tested helper.

Verification

• cargo fmt --all -- --check — passed
• cargo check -q — passed
• cargo build -q — passed
• cargo test -q -p jett_lsp — passed (4 tests)
• cargo test -q — passed (all workspace tests)
• git diff --check — passed

Risk

• Low: document-close handling now publishes the standard empty diagnostic list; compilation and existing LSP behavior are unchanged.

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

[vycdev]

Clearing diagnostics on document close is correct and the LSP tests passed, but this branch now conflicts with current main after the UTF-16 LSP position work in #88 landed. Please rebase without regressing the scalar/UTF-16 conversions, then rerun the full jett_lsp tests. The complementary #121/#126 changes must be serialized afterward because they touch the same backend state.