fix: add missing request body type

[vonovak]

_contentAvailable is documented at https://docs.expo.dev/push-notifications/sending-notifications/#message-request-format
data is a JSON object so keys should be of type string (this is a minor change)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b470314) to head (7ef3496).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #102   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          168       168           
  Branches        33        32    -1     
=========================================
  Hits           168       168           

Flag	Coverage Δ
node_18	100.00% <ø> (ø)
node_20	100.00% <ø> (ø)
node_22	100.00% <ø> (ø)

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