fix: stop OperationHandler spawning extra tasks

[Bravo555]

Proposed changes

When receiving multiple MQTT messages on a command topic, OperationHandler::handle method spawned extra tokio tasks, because .unwrap_or and not .unwrap_or_else was used when checking if a task for that topic already existed. Fortunately because returned object, and along it the sender to the task, was immediately dropped, the task exited and so we weren't leaking anything.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.0%. Comparing base (b79f03a) to head (be3b850).
Report is 88 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
...xtensions/c8y_mapper_ext/src/operations/handler.rs	95.5% <100.0%> (ø)

... and 2 files with indirect coverage changes

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
491	0	2	491	100	1h21m53.26477s

[reubenmiller]

Approved (after verifying the Rust docs)