Preserve structured-only MCP error payloads

[sylvesterkaczmarek]

Summary

Preserve MCP error details when a CallToolResult contains structuredContent but no regular content blocks.

_convert_tool_result() currently handles isError before the existing structuredContent fallback. As a result, an MCP result such as content=[], structuredContent={...}, isError=True raises ToolError([]) and discards the only useful error payload. The tool runner then surfaces a generic Tool error instead of the structured details returned by the MCP server.

Fix

Read structuredContent before branching on isError and use it as the error fallback only when normal content is empty.

Precedence remains:

• explicit MCP content blocks win when present;
• otherwise structured error content is JSON-encoded and preserved in ToolError;
• an error with neither form keeps the existing empty-error behavior;
• successful structured-only results keep the existing JSON-string fallback.

Regression coverage

Adds focused tests covering:

• structured-only MCP errors preserve their payload;
• explicit error content still takes precedence over structuredContent;
• structured-only successful results remain unchanged.

The production change is confined to the hand-maintained MCP tool-result conversion helper.