Fix setpoint_change_source_timestamp not being initialized

[TheJulianJES]

Proposed change

This adds setpoint_change_source_timestamp to ZCL_INIT_ATTRS for the thermostat cluster.
Otherwise, the "Timestamp" entity will be created for every thermostat, even if the device returns UNSUPPORTED_ATTRIBUTE when it's read.

Additional information

This was missed in:

• Add Timestamp Sensor #224

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.63%. Comparing base (696ab1c) to head (11c4c01).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #732   +/-   ##
=======================================
  Coverage   97.63%   97.63%           
=======================================
  Files          62       62           
  Lines       10814    10814           
=======================================
  Hits        10558    10558           
  Misses        256      256           

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[Copilot]

Pull request overview

Adds missing thermostat attribute initialization so entity discovery can correctly suppress the “Timestamp” sensor when the device reports the attribute as unsupported.

Changes:

• Add Thermostat.AttributeDefs.setpoint_change_source_timestamp to ThermostatClusterHandler.ZCL_INIT_ATTRS so it is read during handler initialization and can be marked unsupported early.

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