fix(chat): cache language selection to prevent listener leak in chat editing snapshots (#309905)

[maruthang]

What

Prevents a listener leak triggered by ChatEditingSession.getSnapshotModel during long agent sessions.

Why

getSnapshotModel passed a fresh ILanguageSelection (from LanguageService.createByFilepathOrFirstLine) into IModelService.createModel for every snapshot. Each resulting TextModel internally subscribed (via the selection's observableFromEvent + Event.fromObservable) to the shared LanguageService.onDidChange. In long-running agent sessions the 200-listener threshold was crossed, producing the [Unhandled Error] potential listener LEAK detected telemetry reported in #309905.

How

Resolve the languageId eagerly and pass a static { languageId, onDidChange: Event.None } selection into createModel — the language of a snapshot doesn't change, so there's nothing to subscribe to. This mirrors the pattern already used elsewhere in the codebase (chatTextEditContentPart, terminal.environmentChanges, aiCustomizationManagementEditor).

Test plan

• Added regression test getSnapshotModel does not leak LanguageService.onDidChange listeners, #309905 in src/vs/workbench/contrib/chat/test/browser/chatEditing/chatEditingService.test.ts. The test spies languageService.onDidChange, creates 25 snapshot models, and asserts that new subscriptions are fewer than the number of snapshots (previously it was 1:1).
• npm run compile-check-ts-native passes cleanly for the changed files.

Fixes #309905

[vs-code-engineering]

📬 CODENOTIFY

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@jrieken

Matched files:

• src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingSession.ts