Prevent duplicate poll tasks in pollable sensors

[dmulcahey]

This pull request improves the lifecycle management of polling tasks for PolledElectricalMeasurement sensors in the ZHA integration. The main focus is to prevent orphaned or duplicate polling tasks by making polling enable/disable operations idempotent and ensuring completed tasks are properly cleaned up. Additionally, new tests have been added to validate these behaviors.

Polling task lifecycle improvements:

• Updated maybe_start_polling() in zha/application/platforms/sensor/__init__.py to avoid creating duplicate polling tasks and to remove completed tasks from _tracked_tasks before starting a new one. This ensures only one active polling task exists and prevents stale task accumulation.
• Modified disable() in zha/application/platforms/sensor/__init__.py to safely remove the polling task from _tracked_tasks before cancelling, preventing orphaned tasks and ensuring proper cleanup.

Test coverage enhancements:

• Added test_pollable_sensor_enable_non_idempotent_disable_leaves_orphan_poll_task and test_pollable_sensor_replaces_completed_polling_task to tests/test_sensor.py to verify that enabling/disabling sensors does not leak or duplicate polling tasks, and that completed tasks are replaced and cleaned up as expected.## Summary
  Prevents duplicate poll task creation for pollable sensors on repeated enable calls and hardens disable cleanup.

Failing tests addressed

• tests/test_sensor.py::test_pollable_sensor_enable_non_idempotent_disable_leaves_orphan_poll_task

Verification

• pytest (branch-local targeted run): listed test passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.51%. Comparing base (9d03d63) to head (1e10bd5).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #693   +/-   ##
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  Coverage   97.51%   97.51%           
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  Files          62       62           
  Lines       10949    10956    +7     
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+ Hits        10677    10684    +7     
  Misses        272      272           

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