Add CORS middleware to Starlette app for frontend support

[SmartManoj]

Introduces CORS middleware to the Starlette app instances in FastMCP to allow requests from local web frontends. This enables compatibility with web clients running on localhost and related origins, supporting credentials and common HTTP methods and headers.

Motivation and Context

Fixes #187 (comment)

How Has This Been Tested?

Breaking Changes

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

[dsp-ant]

I don't think we want to add this by default. If we do want to allow for certain types of starlette middlewares. We likely want to mimic what FastMCP 2.0 does here. We want support for a proper middleware stack and a default set for auth, with a pre-defined order.

[SmartManoj]

I don't think we want to add this by default.

Is this wrong?

punkpeye/fastmcp@fe39e7a

[felixweinberger]

I don't think we want to add this by default.

Is this wrong?

punkpeye/fastmcp@fe39e7a

That's looking at the Typescript implementation of FastMCP - if you look at FastMCP 2.0 @dsp-ant was referring to here: #1245

That shows the middleware support in the Python SDK. Tentatively going to mark this as a v2 idea that we'd like to have for tracking

[felixweinberger]

Converting this to a draft for now to remove it from the review queue - keeping this around for future work on v2