docs: clarify client tool cancellation guidance

[anhtnt90dev]

Summary

• Clarify that server-side ctx.request_id is for correlation/protocol handlers, not a public cancellation handle for regular session.call_tool() calls.
• Point long-running cancellable client workflows to task-based cancellation with session.experimental.call_tool_as_task() and cancel_task().
• Note that protocol-level CancelledNotification is for low-level clients that track their own in-flight request IDs.

Closes #1410.

Verification

• uv run --frozen mkdocs build --strict
• uv run --frozen python scripts/update_readme_snippets.py --check --readme README.v2.md
• pre-commit run markdownlint --files README.v2.md
• git diff --check

[maxisbey]

Thanks for the PR. The v2 rework changed the code this targets, so it no longer applies as-is. Closing this as part of a wider backlog cleanup around the v2 release. If it's still relevant, feel free to reopen.

_{AI Disclaimer}