feat: add tool metadata in FastMCP.tool decorator

[mat-octave]

Adds a feature to add metadata to a tool using the FastMCP.tool decorator, passing it to the Tool object.

Motivation and Context

ChatGPT's App SDK expects information about UI resources to be in a tool's _meta field. The decorator FastMCP.tool didn't accept metadata but the field exists on the lower-level mcp.Tool object.

How Has This Been Tested?

I added metadata to mcp.tool(...) and started the server successfully. I used the Model Inspector to list the tools, and the metadata was visible in the schema.

Breaking Changes

No, this just adds an optional field meta.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context