test: Adds test for streaming + function calls

This commit includes a number of new tests for live streaming with function calls. These tests cover various scenarios:
- Single function calls
- Multiple function calls
- Parallel function calls
- Function calls with errors
- Synchronous function calls
- Simple streaming tools
- Video streaming tools
- Stopping a streaming tool
- Multiple streaming tools simultaneously

The tests use mock models and custom runners to simulate the interaction between the agent, model, and tools. They verify that function calls are correctly generated, executed, and that the expected data is returned.

PiperOrigin-RevId: 781318483

Author: hangfei