Remove invalid WARN message for missing notification handler in stateless servers

[bperhaps]

Motivation and Context

#777

According to the MCP specification, notifications from client to server are allowed.

In the case of notification/roots/list_changed, if the client supports listChanged, sending this notification is MUST.
The initialized notification is part of the lifecycle specification.

Since the MCP spec does not define any specific error-handling requirements for notification/roots/list_changed on the server side, it seems more appropriate to accept the notification rather than return a "Missing handler" error.

Because the server is stateless, we have implemented notification/roots/list_changed as a no-op.

How Has This Been Tested?

I wrote test code.

Breaking Changes

nop

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

[Kehrlann]

Thanks for your contribution! This is directionally good, but the tests need reworking.

Both the tests in this PR pass even without your changes, because of the server's default handleNotification which does return Mono.empty.

The only difference with and without your code is suppressing a WARN log message.
Also, there's no need to use mock requests, as the SDK's client can trigger the notifications.

Please update your test to:

1. Use the bundled client
2. Read from the logs and check the absence of the specific warn message

Please also consider adding an appropriate log message for roots/list_changed. After all the client is sending a notification we can't handle, we should at least log it, probably at the DEBUG level, not WARN.

[Kehrlann]

Polished and merged, thanks for your contribution!