feat(bedrock): add native structured output support

[layertwo]

Description

Bedrock now supports native structured output for supported models, enforcing JSON schema compliance at the token generation level via outputConfig.textFormat. Currently, Strands uses a tool-based workaround for structured_output() - converting the Pydantic model into a fake tool spec and forcing tool use. This is indirect and can lead to extra round-trips when the model doesn't format correctly.

Native structured output eliminates the workaround by letting Bedrock guarantee schema-compliant JSON responses directly.

Public API Changes

New structured_output_mode config option on BedrockModel:

# Before: tool-based structured output (still the default)
model = BedrockModel(model_id="us.anthropic.claude-sonnet-4-5-20250929-v1:0")

# After: opt-in to native structured output
model = BedrockModel(
    model_id="us.anthropic.claude-sonnet-4-5-20250929-v1:0",
    structured_output_mode="native",
)

agent = Agent(model=model)
result = agent("Describe the weather in Seattle", structured_output_model=WeatherReport)
weather = result.structured_output

New public utility convert_pydantic_to_json_schema() exported from strands.tools.structured_output — converts a Pydantic model to a JSON schema dict with additionalProperties: false injected recursively, suitable for Bedrock's outputConfig.textFormat.structure.jsonSchema.

No breaking changes. Default behavior (structured_output_mode="tool") is unchanged.

Use Cases

• Schema-guaranteed responses: Native mode enforces JSON schema at the token generation level, eliminating malformed output
• Reduced latency: No extra round-trips from tool-use forcing when the model doesn't comply on the first attempt
• Cleaner agent interactions: No fake tool registration or tool call artifacts in the conversation history

Related Issues

Resolves #1652

Documentation PR

strands-agents/docs#749

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• Unit tests for convert_pydantic_to_json_schema() covering basic, nested, optional, and list-of-objects schemas
• Unit tests for _format_request() with output_config
• Unit tests for structured_output() in native mode (mocked Bedrock responses)
• hatch run prepare passes across Python 3.10-3.14
• Manual testing with live Bedrock API (Claude Sonnet 4.5) confirming native mode works at both agent and model level
• 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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