fix(bedrock): disable thinking mode when forcing tool_choice

[strands-agent]

Description

Fixes #1485 - Cannot use structured_output with thinking enabled.

Problem

When using BedrockModel with thinking mode enabled via additional_request_fields, if structured_output fails to generate a tool call on the first attempt, the event loop forces tool_choice (set to {"any": {}}) to retry. However, Bedrock's API doesn't allow thinking mode to be enabled when tool_choice forces tool use, resulting in:

ValidationException: The model returned the following errors: Thinking may not be enabled when tool_choice forces tool use.

Solution

This PR modifies the _format_request method in BedrockModel to temporarily remove the thinking configuration from additionalModelRequestFields when tool_choice is forcing tool use (i.e., not {"auto": {}}).

Key changes:

1. Detect when tool_choice is forcing a tool (not auto and not None)
2. Create a copy of additional_request_fields without the thinking key when forcing
3. Preserve thinking mode for all normal agent interactions (when tool_choice is auto or None)
4. Preserve all other additional_request_fields when removing thinking

Testing

Added comprehensive unit tests in test_bedrock_thinking_structured_output.py covering:

• ✅ Thinking removed when forcing tool use ({"any": {}})
• ✅ Thinking preserved with auto tool choice ({"auto": {}})
• ✅ Thinking preserved when no tool choice (None)
• ✅ Other additional fields preserved when thinking is removed
• ✅ Models without thinking config work normally

Behavior

Before: Agent crashes with ValidationException when structured_output + thinking enabled
After: Agent gracefully retries structured_output by temporarily disabling thinking for that specific retry

Related Issues

Fixes #1485

Type of Change

• Bug fix

Testing

• Added unit tests
• Logic verified with inline tests

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective
• My changes generate no new warnings
• This is a non-breaking change (thinking mode still works for normal interactions)