[HVAC] Thermostat deadband handling (#35673)

* [HVAC] Shift heating and cooling setpoints to preserve deadband, when possible

* Minor function renames, fix incorrect setters

* Populate deadband on setpointLimits when Auto is supported

* [Thermostat] Add test for deadband handling

* Restyled Python test

* Fix int promotion errors on some platforms

* Remove unused Python imports

* Fix capitalization of structs, methods, fields, etc.

* Silly C++ int promotion rules

* [HVAC] Fix incorrect naming of TSTAT 2.3 test class

* Fix errant quote in TC_TSTAT_2_3.py

* Drop deadband fix test in favor of separate PR

* Apply suggestions from code review

Co-authored-by: Boris Zbarsky <[email protected]>

* Don't use three-arg setter for attributes

* Rename deadband member

* Add backwards-compatibility comment for return code substitution

* Restyled

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Co-authored-by: Boris Zbarsky <[email protected]>