Strands BidiAgent support Nova 2 Sonic expert tool

[lanazhang]

Description

Adds Expert Tool support to BidiNovaSonicModel, enabling bring-your-own-LLM reasoning while Nova Sonic handles ASR and TTS.

New module: strands/experimental/bidi/expert_tool/

• ExpertToolReasoner — protocol for custom reasoning implementations
• BedrockConverseReasoner — built-in reasoner wrapping any Bedrock Converse model with automatic tool calling loops
• StrandsAgentReasoner — wraps any Strands Agent as a reasoner (OpenAI, Gemini, Ollama, etc.)
• ExpertToolConfig — advanced configuration (streaming strategy, interruption callbacks, fallback messages)
• _ExpertToolManager — internal lifecycle manager handling content containers, streaming, barge-in cancellation, and error recovery

Changes to BidiNovaSonicModel:

• New reasoner parameter (shorthand) and expert_tool parameter (advanced config)
• Registers ExpertTool spec in promptStart when reasoner is configured
• Intercepts ExpertTool toolUse events and routes to manager instead of agent loop
• Validates Expert Tool is only used with Nova Sonic v2
• Shuts down manager on stop()

Related Issues

N/A

Documentation PR

NA

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce new warnings.

• 90 unit tests covering config, reasoner (protocol compliance, message conversion, tool calling loops, streaming, error handling), manager (invocation lifecycle, barge-in cancellation, content flags, event helpers), and nova_sonic integration (init validation, spec injection, event interception, stop cleanup)

• All 31 existing test_nova_sonic.py tests continue to pass

• Manual end-to-end testing with Qwen3 32B, Nova 2 Lite, and Claude Sonnet as reasoning LLMs — verified multi-turn conversation, tool calling, and barge-in handling

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• 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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