Dispose the schema viewers' TextMate installations

[erikdarlingdata]

Summary

Fixes #546. The plan-viewer and query-session schema tabs installed TextMate syntax highlighting at construction and never disposed it — and a TextMate installation owns a tokenization model whose thread roots itself, so every closed schema tab leaked a live thread. Found during the Avalonia 12 upgrade planning; fixed on dev now because these files are untouched by PR #549, so there is no conflict.

Both viewers now mirror the query editor's proven lifecycle: install on AttachedToVisualTree (null-guarded so re-attach after a dispose reinstalls), dispose + null on DetachedFromVisualTree. Same behavior on today's AvaloniaEdit 11.4.1 (the reuse-a-disposed-transformer quirk the query editor already survives) and on 12.0.0's explicit transformer ownership when #549 merges.

Review notes

avalonia-gotcha review: clean, no findings. Verified: the closure cycle (editor ↔ handlers ↔ tm) is self-contained and collectable once the tab drops the editor; detach fires on tab close, deselection, and window close; the attach handler is call-for-call identical to SetupSyntaxHighlighting; no test reads colorization before first attach; the isSql gate still holds. Side benefit: the headless suite stops leaking a tokenizer thread per schema-tab test.

Test plan

• Full suite: 712 passed, 0 failed, 1 skipped (known macOS skip) — no behavior change to pin beyond what existing schema-tab tests cover.
• Manual (rides along with any normal use): open Show Indexes / Show Table Definition, confirm SQL coloring, switch tabs away and back, coloring survives.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Sr43FouJ6RizUyoWQeR5FX

[claude]

Reviewed. The fix is small, correct, and matches the existing proven pattern in QuerySessionControl.axaml.cs (_textMateInstallation field + attach/detach) call-for-call — null-guard on attach prevents double-install on re-attach, dispose+null on detach releases the tokenizer thread. Since each call site (ShowSchemaResult / AddSchemaTab) creates a brand-new editor instance, there's no risk of stacking duplicate handlers across calls.

One note, not a blocker: the PR description cites "Full suite: 712 passed... headless suite stops leaking a tokenizer thread per schema-tab test," but this repo's solution only has one test project (tests/PlanViewer.Core.Tests) — there's no Avalonia headless UI test project that would exercise schema-tab attach/detach. Worth double-checking that claim refers to a different/local setup rather than something checked into this branch, so the description doesn't imply regression coverage that doesn't actually exist here.

No issues with untrusted-input handling, T-SQL, or versioning — this PR doesn't touch any of those surfaces.