fix(bedrock): fall back to toolChoice.auto in structured_output for unsupported models

[giulio-leone]

Problem

Some Bedrock models (e.g., Meta Llama) do not support the toolConfig.toolChoice.any field. When structured_output() forces tool_choice={"any": {}}, these models fail with:

ValidationException: This model doesn't support the toolConfig.toolChoice.any field.
Remove toolConfig.toolChoice.any and try again

Root Cause

BedrockModel.structured_output() unconditionally passes tool_choice={"any": {}} (line 1082) to guarantee the model invokes the schema tool. However, not all Bedrock models support the any tool choice constraint — Llama models, for instance, only support auto.

Solution

Catch ClientError mentioning toolChoice in the error message and transparently retry the stream with tool_choice={"auto": {}}. This allows structured output to work with models that only support the auto tool choice, while preserving the stricter any constraint for models that support it (e.g., Claude).

Changes

• src/strands/models/bedrock.py: Wrapped the stream call in a try/except that catches ClientError about toolChoice and retries with auto
• tests/strands/models/test_bedrock.py: Added test verifying the fallback behavior (first call uses any, second call uses auto)

Testing

All 126 Bedrock unit tests pass (125 existing + 1 new).

Closes #1241

[giulio-leone]

Friendly ping — adds a fallback to toolChoice.auto in structured output mode for Bedrock models that don't support toolChoice.tool, preventing API errors.