fix(vscode): reliable cancel and preserved session model on attach

[wbxl2000]

Related Issue

No issue — the problem is explained in the next section (reported by extension users in feedback).

Problem

Two reported correctness problems in the VS Code extension (split out of #1839 to land the bug fixes promptly; the conversation-undo feature stays in that PR):

1. "Stop" sometimes does nothing. SessionRuntime.cancel() returned early whenever the host-side bookkeeping showed no active work. After an abnormal error path, host and engine state can drift apart — the UI shows a running turn while the host thinks the session is idle, so Stop silently never reaches the engine. Additionally abortChat returned { aborted: true } even when no runtime existed, so the UI claimed the task was stopped while nothing was cancelled.
2. Resumed sessions get their model overwritten. Opening or resuming a session force-applied the globally configured default model, stomping a model the user had picked for that session (e.g. via the CLI over the shared session storage). Thinking effort had the same bug and was fixed earlier; model is the symmetric case.

What changed

• Cancel is unconditional (session-runtime.ts): the early return only skips when the runtime is closed — session.cancel() is always invoked (a harmless no-op when the engine is idle), so Stop always converges the engine and the UI even if host bookkeeping drifted. abortChat returns aborted: false when the view has no runtime instead of pretending.
• Session model is preserved on attach (kimi-runtime.ts): the configured default model is only applied at session creation, matching the thinking-effort fix and CLI/TUI resume semantics.
• Composer-submitted model/effort is applied before the turn starts (chat.handler.ts): since attach no longer syncs modes, the streamChat preflight applies the model/effort sent with the prompt (previously a picker change followed immediately by Send could race onto the old model). resume-vs-prompt semantics stay distinct: reopening preserves the session's modes, sending applies what the composer shows.

Tests: idle-cancel still reaching the engine, abortChat honesty at the bridge boundary, preserved-model on resume, and an end-to-end integration test that submits a different model/effort with a prompt and asserts the session adopts them.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset. (No changeset: vscode-extension-only change, nothing enters the CLI bundle.)
• Ran gen-docs skill, or this PR needs no doc update. (Extension behavior fix; no user-facing docs affected.)

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