POC: support input stream execution

[kalenkevich]

Description:

• Implemented runStream in Runner which accepts an AsyncIterable<Content> for continuous input streaming.
• Developed a concurrent multiplexing model that consumes the user stream, injects events into the SessionService, and triggers or extends the runAsync agent loop dynamically.
• Introduced an AsyncEventQueue internal helper to synchronize and yield multiplexed input events (from the user stream) and generated events (from the model) as a single streaming output.
• Added a demonstration in samples/streaming_input_agent.ts showcasing a weather agent interacting with time-delayed stream inputs.

Check example: samples/streaming_input_agent.ts