Unify model and session interface

[mkmeral]

Description

This PR implements a unified bidirectional streaming interface that simplifies the API while maintaining strong typing and provider flexibility. Following the decision from PR #11, we've adopted a unified model interface that combines configuration and connection state in a single class, eliminating the separate session abstraction.

Key Changes

1. Unified Model Interface

Before (Two-Layer):

model = NovaSonicBidirectionalModel(region="us-east-1")
session = await model.create_bidirectional_connection(system_prompt="...")
await session.send_text_content("Hello")
await session.receive_events()
await session.close()

After (Unified):

model = NovaSonicBidirectionalModel(region="us-east-1")
await model.connect(system_prompt="...")
await model.send("Hello")
await model.receive()
await model.close()

2. Unified send() Method

Replaced multiple content-specific methods with a single polymorphic send() method:

Before:

await session.send_text_content("Hello")
await session.send_audio_content(audio_event)
await session.send_image_content(image_event)
await session.send_tool_result(tool_id, result)
await session.send_interrupt()

After:

await model.send(TextInputEvent(text="Hello", role="user"))
await model.send(AudioInputEvent(audioData=bytes, format="pcm", ...))
await model.send(ImageInputEvent(imageData=bytes, mimeType="image/jpeg"))
await model.send(ToolResult(toolUseId="123", status="success", ...))

3. New Typed Input Events

Added TextInputEvent for consistency with other input event types:

class TextInputEvent(TypedDict):
    text: str
    role: Role

4. Simplified Connection Lifecycle

• BidirectionalModel now directly implements connect(), send(), receive(), and close()
• Removed BidirectionalModelSession as a separate abstraction
• Added backwards compatibility alias: BidirectionalModelSession = BidirectionalModel

5. Updated All Provider Implementations

• Nova Sonic: Unified interface with internal state management
• Gemini Live: Simplified SDK integration without separate session class
• OpenAI Realtime: Streamlined WebSocket connection handling

6. Comprehensive Unit Tests

Added extensive test coverage for all three providers:

• tests/strands/experimental/bidirectional_streaming/models/test_gemini_live.py (500 lines)
• tests/strands/experimental/bidirectional_streaming/models/test_novasonic.py (551 lines)
• Tests cover initialization, connection lifecycle, event conversion, and unified send method

Benefits

✅ Simpler API: Single interface per model, familiar pattern matching core Strands Agent
✅ Better Type Safety: Explicit typed events for all input types
✅ Cleaner Architecture: Internal separation maintained without exposing complexity
✅ Industry Alignment: Matches patterns from Google Gemini Live and OpenAI Realtime APIs
✅ Backwards Compatible: Alias provided for existing code using BidirectionalModelSession

Related Issues

Implements the unified interface decision from #11

Documentation PR

Documentation updates will follow in a separate PR to agents-docs

Type of Change

Breaking change

Testing

• ✅ Added comprehensive unit tests for all three providers (Gemini Live, Nova Sonic, OpenAI Realtime)
• ✅ Tests cover initialization, connection lifecycle, unified send method, event conversion
• ✅ All existing integration tests updated to use new interface
• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[JackYPCOnline]

Overall comment regarding file naming convention: Since these files are already located within the bidirectional_streaming directory, we don't need to include "bidirectional_streaming" in each individual file name, eg we could just call base_model, event_type something.

[JackYPCOnline]

I think you replied me, but here is session_id? bad rebase?

[JackYPCOnline]

also here

[JackYPCOnline]

TODO: This pr is a dependency to other ongoing works. Merge this PR now, need to follow up on naming and coding consistency.