Fix unstructured tool errors being masked as 'outputSchema defined but no structured output returned'

[jairus-m]

Summary

When a tool raised an exception, DbtMCP.call_tool() caught it and returned [TextContent(...)] which is unstructured. After #774 changed structured_output to default to True, all tools gained an outputSchema. Because of this, the MCP server rejects the unstructured error response with "Output validation error: outputSchema defined but no structured output returned". The actual error is not returned and hidden to the user/LLM:

To address this, we instead return CallToolResult(isError=True) on exception so the MCP server "short circuits" raise an error which gets caught by FastMCP's handler before the outputSchema check which then properly outputs the failure:

Checklist

• I have performed a self-review of my code
• I have made corresponding changes to the documentation (in https://github.com/dbt-labs/docs.getdbt.com) if required -- Mention it here
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

One thing Im unsure about about this implementation here is that we have to override the return type established by the parent FastMCP.call_tool method and add CallToolResult to DbtMCP.call_tool's annotations. Because of this, mypy gets a bit mad and we have to tell it to "shhh". No behavioral changes with regards to the LSP, just type annotations! Hope this is fine.

^^ addressed in comments