chore: add ability to remove tools

[brandonspark]

Motivation and Context

I am interested in being able to dynamically have a server offer only certain tools based on information received during its initialization. I could conditionally declare the tool, for instance:

if cond:
  @mcp.tool
  async def foo():
    ...

but I perceive this to be relatively gross. I would prefer to be able to remove a tool within the API.

How Has This Been Tested?

I can achieve comparable behavior by inserting an if before defining a function with the mcp.tool() decorator.

Breaking Changes

No.

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