Implement Runner streaming and stateless execution

[dewitt]

This PR implements the core streaming and session management improvements for the Runner class, ensuring architectural parity with the ADK Python implementation.

Key Features:

• Streaming Execution (runAsync): The primary entry point for streaming. Returns an AsyncGenerator, yielding raw, non-lossy ADK events in real-time as the model processes the request.
• Ephemeral Sessions (runEphemeral): A helper method that creates a temporary session, executes a stream (with an optional initial stateDelta), and automatically cleans up the session data upon completion.
• Structured Introspection (toStructuredEvents): An optional, standalone utility that parses raw ADK events into a strongly-typed list of StructuredEvent objects. It categorizes parts into high-level actions (ThoughtEvent, ContentEvent, ToolCallEvent, CallCodeEvent, etc.) to simplify client-side event handling for complex UI or CLI integrations.

Reference: docs/adk-ts-improvements.md

Fixes #141.

[kalenkevich]

can we please also include some tests for those changes?

[dewitt]

@kalenkevich I think I've resolved all of your comments, and updated the PR description to match the revised API.

PTAL, and thank you!

[kalenkevich]

A bit more comments, sorry

[dewitt]

Thanks for the additional round of comments, @kalenkevich! The code is better for it.

All suggestions and fixes incorporated. PTAL.

[kalenkevich]

Thanks!