Fix/bedrock empty tool result structured content

[Devansh-jpg]

Changes

Fix _mcp_content_to_langchain crashing AWS Bedrock when an MCP server returns an empty content[] with data in structuredContent (MCP 2025-11-05 spec).

Implementation Details

1. Changed _mcp_content_to_langchain to accept a full CallToolResult object instead of just the content list, so it can access both content and structuredContent.
2. When content is empty, fall back to json.dumps(structuredContent) if available, otherwise return "(no content)" — both are non-empty strings that Bedrock accepts.
3. Updated the call site to pass tool_result instead of tool_result.content.

Example Usage (Before)

# MCP server (2025-11-05 spec) returns:
# {"content": [], "structuredContent": {"success": true, "data": {...}}}
# → _mcp_content_to_langchain returns ""
# → Bedrock raises ValidationException: tool_use ids found without tool_result blocks

Example Usage (After)

# Same response from MCP server
# → _mcp_content_to_langchain returns json.dumps({"success": true, "data": {...}})
# → Bedrock accepts it, agent continues normally

Testing

- Updated 3 existing tests to pass CallToolResult instead of a raw list (required by new signature)
- Added test_empty_content_with_structured_content_returns_json — confirms structuredContent fallback works
- Added test_empty_content_without_structured_content_returns_placeholder — confirms "(no content)" is returned instead of "" when both fields are empty
- All 6 tests pass

Backwards Compatibility

No breaking change for existing users. The only behavioral difference is when content[] is empty — previously returned "", now returns structured data or a placeholder. Both are non-empty strings compatible
with all LLM providers.

Related Issues

Closes #1604