fix: use const for max BLE payload size

[2bndy5]

This replaces the magic number which was inconsistent between RX and TX BLE operations.

This also avoids possible malformed outgoing payloads because 28 was previously used where 27 is the correct maximum.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1513ada) to head (aeded67).
Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #30   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         3154      3154           
=========================================
  Hits          3154      3154           

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