private key account tests

[0xFirekeeper]

PR-Codex overview

This PR enhances the PrivateKeyAccount class with improved validation and test coverage for signing transactions and data.

Detailed summary

• Improved validation for null or empty messages in signing methods
• Added tests for initialization, connection, signing methods
• Enhanced error handling for different transaction types and properties

The following files were skipped due to too many changes: Thirdweb.Tests/Thirdweb.PrivateKeyAccount.Tests.cs

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.77%. Comparing base (2ff1a05) to head (caaba7f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   32.76%   37.77%   +5.00%     
==========================================
  Files          33       32       -1     
  Lines        2362     2197     -165     
  Branches      216      197      -19     
==========================================
+ Hits          774      830      +56     
+ Misses       1564     1353     -211     
+ Partials       24       14      -10     

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