fix(balances): return balance as string

[dtfiedler]

For unknown reasons, the MU does not crank the Balance-Notice message back to a calling process when Data is not a string. This updates all handlers that return numbers to return the strings.

My initial thought was that this would cause breaking changes with the SDK - but given our use of JSON.parse - which parses strings to numbers, it actually has no obvious downstream consequences. We will continue to test in devnet before pushing to testnet, specifically with arns.app, network-portal.app and ArConnect.

Related: ar-io/ar-io-sdk#327

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.46%. Comparing base (7bbe777) to head (36a51c7).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #303   +/-   ##
========================================
  Coverage    94.46%   94.46%           
========================================
  Files           10       10           
  Lines         2997     2997           
========================================
  Hits          2831     2831           
  Misses         166      166           

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