[wip] Add canvas launch authorization and session retention

[ulugbekna]

Summary

Add the public Node SDK launch-provider and no-turn retention contracts required by a host that runs explicitly approved local canvas packages.

Included

• CopilotClientOptions.extensionLaunchProvider, public request/result/profile/source types, and exact live { contractVersion: 1 } acknowledgement before creation/resume and on replacement connections.
• Shared startup, cancellation and replacement-connection handling; older/null/error acknowledgements fail closed rather than retrying without the provider.
• Generated session.retain / session.retained bindings and client.retainSession(sessionId) for connected-client retention while an initial create/resume is still waiting on a launch resolver.
• Documentation and focused unit/E2E fixtures covering registration, denial/error, reconnect, retention, reentrant startup and actual package execution.
• Clients without a launch provider preserve their existing behavior.

This API is not a code-integrity mechanism, trust store or backend sandbox. The consuming host still owns exact package/revision/workspace approval.

Validation

• Fresh pre-PR run: npx vitest run test/extension-launch-provider.test.ts — 23 passing.
• The implementation handoff additionally records 93 targeted tests, type/lint/format checks, real compatible-runtime tests and packed-package consumption checks.
• The compatible stack was exercised through actual native canvas creation, actions, lifecycle and canvas-originated model requests.
• The generated protocol files came from the runtime's canonical schema generation, not manual mirror edits.
• Generation reflects the complete supplied schema snapshot, including unrelated upstream declaration changes; it does not imply additional SDK feature implementation or compatibility coverage.

WIP / dependencies

• Requires a companion compatible runtime implementation; registration with an incompatible runtime must reject.
• The previously reproduced model-selection/disconnect/same-ID-resume failure is fixed in the companion runtime. The unchanged public SDK reproducer and full three-test host integration file now pass with that corrected runtime; no additional SDK API change was needed.
• This draft does not bump/release SDK packages or claim all-language SDK parity for the new Node client option.

Local .local/ and nodejs/.local/ caches, packed artifacts, runtime logs and consumer scratch directories are intentionally excluded.

Public companion drafts

• [wip] Add local canvases to the Agents Window microsoft/vscode#335902 — consuming Agents Window implementation.
• [wip] Add local-canvases protocol contract (LC-17) microsoft/agent-host-protocol#443 — renderer-neutral host protocol.

The runtime implementation is tracked separately in its existing private repository.