docs: explain FastMCP request IDs for cancellation

[lavish0000]

Closes #1410

Summary:

• document that ctx.request_id and ctx.request_context.request_id expose the current FastMCP request ID inside a tool
• clarify that notifications/cancelled uses that same JSON-RPC request ID for normal in-flight requests such as tools/call
• note the current limitation that high-level client helpers do not expose request IDs directly, and point readers toward task-based cancellation for explicit client-side control

Testing:

• python3 -m uv run pytest tests/shared/test_session.py -k test_request_cancellation