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/ui/commands/restoreCommand.test.ts

@@ -48,6 +48,7 @@ describe('restoreCommand', () => {
         getProjectTempDir: vi.fn().mockReturnValue(geminiTempDir),
       },
       getGeminiClient: vi.fn().mockReturnValue({
+        getSessionPrimaryAgent: vi.fn().mockReturnValue(null),
         setHistory: mockSetHistory,
       }),
     } as unknown as Config;