fix: Remove flaky doctest

[rmccorm4]

Overview:

This doctest has been failing intermittently, causing rust test failures on various PRs, for example: https://github.com/ai-dynamo/dynamo/actions/runs/18419132132/job/52489609655?pr=3539

It is relatively rare, maybe 1 out of 10-20 runs that it will fail, but it's enough to make the CI test unreliable. This should be revisited and fixed properly to determine if there is an actual bug in the underlying implementation or just an issue with the doctest example.

Details:

For future reference: I had an attempt to fix here, but I'm not confident in its correctness: #3414

I'm opting to remove the flakiness for now to stabilize CI, and have this revisited independently.

Summary by CodeRabbit

• Documentation
  • Updated inline documentation by removing an unstable example to prevent intermittent failures in automated checks. No impact on behavior or APIs.
• Chores
  • Maintenance to improve reliability of documentation-related validation during builds and tests. No functional changes for end-users.

[coderabbitai]

Walkthrough

Replaced an inline doctest in TaskHandle::cancellation_token() with a FIXME comment citing intermittent doctest failures. No code logic, signature, or control flow was modified.

Changes

Cohort / File(s)	Summary
Docs/Doctest adjustment lib/runtime/src/utils/tasks/tracker.rs	Removed doctest example from TaskHandle::cancellation_token() and added a FIXME comment about flaky doctests; no functional changes.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

I thump my foot at flaky tests—oh dear!
A doctest hopped away, we’ll fix it here.
With FIXME carrots planted neat,
The code still hums, serene and sweet.
Until the tests return to play—hip hop, hooray! 🥕🐇

Pre-merge checks

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.
Description Check	⚠️ Warning	The PR description includes Overview and Details sections as per the template but omits the 'Where should the reviewer start?' and 'Related Issues' sections that are required by the repository’s description template. Without guidance on which files to review and the linkage to an existing issue via an action keyword, reviewers may be uncertain where to focus and how this PR relates to tracked issues. This omission makes the description incomplete and fails to meet the template’s structure.	Please add a 'Where should the reviewer start?' section that specifies the key files or areas for review and add a 'Related Issues' section using an action keyword (e.g., closes #xxx) to link this PR to the relevant issue. This will ensure reviewers have guidance on where to focus and that the PR is properly associated with issue tracking.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The title succinctly states the core change of removing a flaky doctest and aligns directly with the implemented modifications, making it clear and specific.

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[rmccorm4]

CC @ryanolson for viz since you wrote these doctests - just trying to stabilize CI for now, couldn't come up with a confident fix atm.