extensions: show installed agent plugins when searching @agentPlugins @installed (#302530)

* extensions: show installed agent plugins when searching @agentPlugins @installed

Handle @agentPlugins @installed query in the agent plugins view by treating
@installed as a filter modifier rather than search text. Also prevent @installed
modifier from triggering the default installed extensions view when @mcp or
@agentPlugins queries are present.

- Extract @installed from search text in AgentPluginsListView.show() and use
  it as a filter to exclude marketplace plugins from results
- Exclude @mcp and @agentPlugins queries from isInstalledExtensionsQuery and
  isSearchInstalledExtensionsQuery detection methods to prevent incorrect view
  routing
- Add test cases to verify that @mcp and @agentPlugins queries with @installed
  are not treated as installed extension queries

Fixes https://github.com/microsoft/vscode/issues/302509

(Commit message generated by Copilot)

* fix