Fixes protocol compliance gap described in issue #1419

[ChenyangLi4288]

Brief summary of changes

• Add RequestResponder.mark_cancelled_without_response() to support silent cancellation.
• Use silent cancellation in BaseSession._receive_loop() when handling CancelledNotification.
• When emitting a CancelledNotification, proactively cancel the local pending request so send_request() unblocks with McpError("Request cancelled").

Motivation and Context

• Aligns with MCP spec guidance: receivers of cancellation notifications SHOULD not send a response.
• Fixes protocol compliance gap described in issue #1419 (link).

How Has This Been Tested?

• Ran tests/shared/test_session.py::test_request_cancellation to confirm the client’s in-flight request is cancelled promptly without waiting for a peer response.
• Sanity-checked other session paths to ensure no regressions; linter clean.

Breaking Changes

• None. Behavior change only affects cancellation-notification flows and makes them spec-compliant.

Types of changes

• [x] 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

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

Additional context

• Implements silent cancellation per spec and avoids emitting JSON-RPC error responses upon receiving CancelledNotification, improving interoperability and reducing unnecessary traffic.

[ChenyangLi4288]

any chance of getting review? @felixweinberger