feat(cli): A2A Server Primary Agent Support

- Added a new `/agents use <name>` command to allow users to set a remote agent as the primary agent for the current session.
- Implemented `A2AStreamingAdapter` to seamlessly integrate A2A server task polling with the existing CLI streaming UX.
- Mapped A2A server task status updates to Gemini `Thought` events, providing native thinking visualization in the terminal.
- Updated `nonInteractiveCli` and `useGeminiStream` to dynamically route requests through the `A2AStreamingAdapter` when a session primary agent is configured.
- Ensured context history continuity by maintaining `contextId` and `taskId` across requests within the adapter.

packages/cli/src/config/extension-manager-themes.spec.ts

@@ -93,6 +93,7 @@ describe('ExtensionManager theme loading', () => {
         startExtension: vi.fn().mockResolvedValue(undefined),
       }),
       getGeminiClient: () => ({
+        getSessionPrimaryAgent: vi.fn().mockReturnValue(null),
         isInitialized: () => false,
         updateSystemInstruction: vi.fn(),
         setTools: vi.fn(),
@@ -203,6 +204,7 @@ describe('ExtensionManager theme loading', () => {
       setGeminiMdFilePaths: vi.fn(),
       getEnableExtensionReloading: () => true,
       getGeminiClient: () => ({
+        getSessionPrimaryAgent: vi.fn().mockReturnValue(null),
         isInitialized: () => false,
         updateSystemInstruction: vi.fn(),
         setTools: vi.fn(),