fix(site): dispose Monaco diff models on unmount (#28503) - #28809

Backport of #28503 to release/2.37.

`SyntaxHighlighter` sets `keepCurrentOriginalModel` /
`keepCurrentModifiedModel`, so `@monaco-editor/react` never disposes the
diff editor's text models. Without stable model paths, every render of a
template version created fresh anonymous models that were never freed,
so navigating between versions leaked models until the tab hung.

Move the diff editor into a `DiffFile` component that captures the
original and modified models in `onMount` and disposes them when the
diff editor unmounts, including the diff to plain switch while
`SyntaxHighlighter` stays mounted. Disposal is deferred with
`queueMicrotask` because freeing the models in the same synchronous
teardown throws `TextModel got disposed before DiffEditorWidget model
got reset`. The `keepCurrent*Model` flags stay, since removing them
reintroduces that mid-teardown throw.

Adds `SyntaxHighlighter.stories.tsx` with a `DisposesModelsOnDiffToggle`
play test asserting the model count returns to baseline across diff and
plain toggles.

(cherry picked from commit 5008603)