chat: persist installed plugin identity

[connor4312]

chat: persist installed plugin identity

Persists enough installed plugin identity for marketplace plugins to survive reloads while keeping installed.json easy for external tools to read.

• Stores plugin name alongside plugin URI and marketplace in installed.json, avoiding full source descriptor persistence.
• Hydrates installed plugin metadata by re-reading marketplace data and matching current entries by stored name.
• Falls back to install URI matching for older entries without names and keeps package/git install URI handling consistent.
• Adds focused coverage for GitHub-sourced marketplace plugins like azure from awesome-copilot.

(Commit message generated by Copilot)

[Copilot]

Pull request overview

This PR updates the chat plugin installation persistence model so marketplace-installed plugins can be rehydrated correctly after a reload/restart, without persisting full plugin descriptors in installed.json.

Changes:

• Persist installed plugin name alongside pluginUri and marketplace in installed.json.
• Update hydration logic to re-read marketplace metadata (including GitHub-shorthand marketplaces via cached fetch) and resolve installed plugins by stored name (with URI fallback for older entries).
• Adjust repository/install URI handling and add/extend tests for restart hydration and GitHub-sourced marketplace plugins.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/common/plugins/pluginMarketplaceService.ts	Store plugin name in installed entries and hydrate metadata after restart using marketplace data.
src/vs/workbench/contrib/chat/common/plugins/fileBackedInstalledPluginsStore.ts	Extend installed.json schema to include optional name and migrate legacy data accordingly.
src/vs/workbench/contrib/chat/browser/agentPluginRepositoryService.ts	Centralize install URI logic and harden relative-path install URI normalization/validation.
src/vs/workbench/contrib/chat/browser/pluginInstallService.ts	Delegate install URI resolution to the repository service.
src/vs/workbench/contrib/chat/test/common/plugins/pluginMarketplaceService.test.ts	Add restart hydration coverage for GitHub-sourced marketplace plugins and name persistence.
src/vs/workbench/contrib/chat/test/common/plugins/fileBackedInstalledPluginsStore.test.ts	Update migration expectations to include persisted plugin name.
src/vs/workbench/contrib/chat/test/browser/plugins/pluginInstallService.test.ts	Update repository stub to align with new install URI delegation behavior.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/test/common/plugins/pluginMarketplaceService.test.ts:578

• This second-session hydration also relies on a fixed retry loop + timeout(10), which can be flaky. Prefer waitForState(second.installedPlugins, p => p.length === 1) or a shared waitFor helper to await the observable update deterministically.

		const second = makeService();
		for (let i = 0; i < 50; i++) {
			if (second.installedPlugins.get().length === 1) {
				break;
			}
			await timeout(10);
		}

• Files reviewed: 7/7 changed files
• Comments generated: 3