Fix FrankenPHP worker mode compatibility

[camilleislasse]

The Protocol keeps a reference to the transport but never resets it after a request. In worker mode, the same Protocol instance is reused across requests, causing "Protocol already connected to a transport" error on subsequent requests.

Add Protocol::disconnect() to reset the transport reference, called in Server::run()'s finally block after transport->close().

Motivation and Context

Fixes #999

How Has This Been Tested?

• Reproduced the issue with FrankenPHP worker mode (symfony/ai demo app)
• Verified fix works (multiple sequential requests succeed)
• Added unit tests for worker mode scenario

Breaking Changes

None

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

[lvluoyue]

Hello, I submitted a similar PR #160 , but he removed the reference to transport, thus achieving the ability to handle multiple transports simultaneously. Is this useful for you?

[camilleislasse]

Hello, I submitted a similar PR #160 , but he removed the reference to transport, thus achieving the ability to handle multiple transports simultaneously. Is this useful for you?

I tested your PR with FrankenPHP worker mode and it works perfectly. Your approach is cleaner - removing the transport reference entirely is better than resetting it. I'll close my PR in favor of yours.