Fix thermostat rounding and Sinope polling idempotency

[dmulcahey]

This pull request addresses two main issues with Sinope thermostat entities: preventing duplicate time update tasks when enabling the entity, and improving temperature setpoint precision. It also includes related test coverage and minor code improvements.

Sinope time update task management:

• Added a check in start_polling() to prevent duplicate time update tasks from being created when enable() is called multiple times. This ensures only one background task is active per entity.
• Updated disable() to safely remove the time update task from _tracked_tasks using contextlib.suppress to avoid errors if the task is already removed.
• Introduced a test (test_sinope_enable_is_idempotent_for_time_update_task) to verify that enabling a Sinope thermostat multiple times does not create duplicate time update tasks.

Temperature setpoint precision:

• Changed all setpoint conversions from int(temperature * ZCL_TEMP) to round(temperature * ZCL_TEMP) in async_set_temperature() to preserve decimal precision and avoid subtle setpoint drift.
• Added a test (test_set_temperature_heat_decimal_precision) to confirm that decimal precision is maintained in setpoint conversions for heating.

Minor improvements:

• Imported contextlib to support safe task removal.

Failing tests addressed

• tests/test_climate.py::test_set_temperature_heat_decimal_precision
• tests/test_climate.py::test_sinope_enable_is_idempotent_for_time_update_task

Verification

• pytest (branch-local targeted run): both tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.51%. Comparing base (9d03d63) to head (4102263).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #688   +/-   ##
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  Coverage   97.51%   97.51%           
=======================================
  Files          62       62           
  Lines       10949    10953    +4     
=======================================
+ Hits        10677    10681    +4     
  Misses        272      272           

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