chat: wait for native metered state before plugin updates

[Dmitriy Vasyura (dmitrivMS)]

Stack

Stacked directly on #331897 (dev/dmitriv/use-vscode-metered). This PR contains only the plugin auto-update portion of the original combined change and does not depend on the other consumer PRs. Merge the primary PR first, then retarget this PR to main.

Changes

• Wait for native metered state before automatically updating plugins.
• Initialize periodic marketplace update checks after the initial state is available, including an unmetered snapshot that emits no change event.
• Gate marketplace update checks on readiness and avoid work after disposal.
• Retain regression tests for plugin update/check initialization.

Only the required typed test-double API migrations remain in the primary PR; plugin behavior and its regression coverage are isolated here.

Validation

• 105 plugin auto-update and marketplace tests passed.
• Client type-check, targeted transpilation, and staged-file hygiene/ESLint passed.
• The diff is limited to four plugin files. Applying all four disjoint sibling diffs to the primary branch exactly reconstructs the original combined PR tree.