Dispose ConversationFeature in sanity test teardown

[benvillalobos]

Replace conversationFeature.activated = false; with conversationFeature.dispose(); in each per-test finally block of the Copilot Chat Sanity Test suite.

The previous teardown only flipped the activated setter, which clears _activatedDisposables (commands, terminal quick-fix providers, participant registrations, etc.) but leaves _disposables intact. _disposables holds the two onDidAuthenticationChange listeners that each test instance subscribes to. Across retries those listeners accumulate on the shared IAuthenticationService, and on an auth-token oscillation they can re-trigger registration of the global copilot-chat.fixWithCopilot terminal quick-fix provider, surfacing as a flaky "already registered" failure.

dispose() is a strict superset of activated = false — it clears both stores. Each ConversationFeature instance's disposables are per-listener / per-registration (verified against IAuthenticationService.onDidAuthenticationChange and vscode.window.registerTerminalQuickFixProvider), so disposing the test-owned instance does not affect the contribution-registered ConversationFeature from the extension activation path.

No test reuses conversationFeature after its finally block.

[Copilot]

Pull request overview

This PR fixes a teardown leak in the Copilot Chat sanity test suite by disposing per-test ConversationFeature instances rather than only deactivating them, preventing accumulation of authentication-change listeners across retries.

Changes:

• Replace conversationFeature.activated = false; with conversationFeature.dispose(); in per-test finally blocks.
• Ensure both _activatedDisposables and lifetime _disposables (e.g., auth listeners) are cleaned up for each test instance.

Show a summary per file

File	Description
extensions/copilot/src/extension/test/vscode-node/sanity.sanity-test.ts	Updates test teardown to dispose ConversationFeature instances to avoid listener/provider re-registration flakiness.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0

[benvillalobos]

Closing as superseded by #316732. That PR drops the per-test ConversationFeature construction entirely, so the four finally blocks this PR was patching no longer exist on main. The leak diagnosis here (activated = false doesn't dispose _disposables / the auth listener) is valid but moot once #316732 lands.