fix(server): expose request cancellation on MCPServer context - #3390

Fixes #3389.

Problem

The high-level MCPServer Context does not expose the cancellation event available in the underlying dispatch context. Tool handlers therefore cannot observe cancellation or perform cooperative cleanup through the public API.

Changes

• Add cancel_requested to ServerRequestContext.
• Forward the dispatcher's existing cancellation event when constructing the request context.
• Expose cancel_requested on the high-level Context.
• Preserve the existing error when context is accessed outside a request.
• Add regression tests for cancellation access and access outside a request.

Testing

• All 168 tests passed:
  python -m pytest tests/server/mcpserver/test_server.py -v --inline-snapshot=disable
• Ruff linting passed.
• Ruff formatting checks passed.

AI assistance disclosure: I used AI to help investigate and implement this change. I reviewed the implementation and verified all tests myself.