fix(bedrock): normalize empty toolResult content arrays in _format_bedrock_messages

[ghhamel]

Description

Some model providers (e.g., Nemotron) reject toolResult blocks with content: [] via the Bedrock Converse API, while others (e.g., Claude) accept them. When a tool returns an empty result (common with MCP tools like awslabs/postgres-mcp-server), the SDK passes the empty content array through to the Converse API unchanged, causing a ValidationException on strict models.

This PR normalizes empty toolResult.content arrays to [{"text": ""}] in _format_bedrock_messages, following the same pattern as:

• The TypeScript SDK's _formatMessages in bedrock.ts, which already filters empty content
• Existing Python SDK normalizations in the same function (SDK_UNKNOWN_MEMBER filtering, DeepSeek reasoningContent dropping)

Related Issues

Fixes #2122

Documentation PR

N/A

Type of Change

Bug fix

Testing

• Added test_format_bedrock_messages_normalizes_empty_tool_result_content — verifies empty toolResult.content is replaced with [{"text": ""}]

• Added test_format_bedrock_messages_preserves_nonempty_tool_result_content — verifies non-empty content is not modified

• All 129 existing test_bedrock.py tests pass with no regressions

• Verified fix against live Bedrock endpoints: Nemotron succeeds with the fix, Claude continues to work unchanged

• 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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