Make light polling and transition cleanup idempotent

[dmulcahey]

Repeatedly enabling a light starts another refresh task and loses the handle used by disable(). Transition cleanup also clears _transition_listener before trying to remove that handle from _tracked_handles, retaining cancelled timers.

Keep the active polling task, discard a completed task before replacement, and remove the original transition handle before clearing it. Regression tests cover repeated enable/disable, completed-task replacement, and timer unsubscription.

Validation against dev at 66603431339afe37fa0048b70ff31d77dceb8f95: Python 3.12 full suite, 1385 passed, coverage above the 95% project gate; full pre-commit (codespell, Ruff, formatting, mypy, lock check) passed. Relevant regression checks fail on the unchanged base. GitHub CI for Python 3.12/3.13/3.14 is reported separately on the PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (6660343) to head (d1aa5c3).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #692   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files          57       57           
  Lines       10560    10567    +7     
=======================================
+ Hits        10264    10271    +7     
  Misses        296      296           

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[Copilot]

🟢 Approval recommended

Changes are narrowly scoped, address the described resource-tracking bugs directly, and are backed by targeted regression tests.

Pull request overview

This PR fixes idempotency issues in the light platform’s background refresh polling and transition timer cleanup, preventing orphaned tasks/handles when enabling/disabling lights repeatedly and ensuring tracked resources are correctly removed.

Changes:

• Make start_polling() idempotent by reusing an active refresh task and removing completed tasks from _tracked_tasks before replacement.
• Fix transition listener cleanup by removing the original timer handle from _tracked_handles before clearing _transition_listener.
• Add regression tests covering repeated enable/disable, completed-task replacement, and transition timer unsubscription.

File summaries

File	Description
zha/application/platforms/light/__init__.py	Ensures refresh polling tasks and transition timer handles are cleaned up deterministically and don’t become orphaned on repeated operations.
tests/test_light.py	Adds regression coverage for repeated enable/disable behavior and correct task/handle tracking cleanup.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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