feat(api): abort signal for vscode-lm, gemini, mistral (SDK-specific quirks) #618

Parent issue

Part of #404 — Stop does not abort underlying HTTP request
Depends on #615 (core plumbing). Can be developed in parallel with #616 and #617.

Scope

3 providers with SDK-specific constraints that each need individual attention.

Provider-specific guidance

vscode-lm.ts — CancellationToken ↔ AbortSignal bridge

VS Code uses its own cancellation model (CancellationToken / CancellationTokenSource) with the disposable pattern. This provider must bridge between the two:

• Create a CancellationTokenSource, wire metadata.abortSignal to call source.cancel() on abort
• Always call source.dispose() in a finally block (see vscode#205966 for the leak this prevents)
• The { once: true } + cleanup pattern from PR feat(api): pass abortSignal to streaming API calls for all providers (#404) #434 is correct here; the redundant removeEventListener inside the { once: true } callback can be removed

gemini.ts — signal placement in config

• abortSignal goes inside the config object — this IS correct per the @google/genai SDK
• Fix: Keep GenerateContentParameters type instead of casting to any. Use config: { ...config, abortSignal: metadata?.abortSignal } as GenerateContentConfig
• Note: per the SDK docs, abortSignal is client-only — Google still charges for server-side compute after abort. Worth a code comment.

mistral.ts — acknowledged gap

• The Mistral non-streaming SDK (chat.complete()) may not support a signal option
• Verify this claim against the actual SDK types/source before accepting it — if incorrect, wire the signal
• If confirmed, add a // TODO with a link to the SDK docs and leave completePrompt as-is

Tests

• vscode-lm.ts: test that CancellationTokenSource.cancel() is called when abort signal fires, and that dispose() is called in both success and error paths
• gemini.ts: test signal is forwarded inside config for both streaming and non-streaming paths
• mistral.ts: test streaming path forwards signal; non-streaming path can have a skip-test with explanation

Acceptance criteria

• vscode-lm.ts properly bridges CancellationToken ↔ AbortSignal with disposal
• gemini.ts keeps GenerateContentParameters type (no any cast)
• mistral.ts gap is verified and documented
• Tests cover all paths