test: add unit tests for core module (LockStatus, MessageType, ProtocolConstants)

[NiMv1]

What is the purpose of the change

This PR adds unit tests for the core module to improve test coverage, addressing issue #6502.

Brief changelog

• Add LockStatusTest.java with tests for:

  • getCode() method
  • get(byte) and get(int) methods
  • Invalid code exception handling
  • Enum values validation

• Add MessageTypeTest.java with tests for:

  • Version and type constants
  • Global transaction type constants
  • Branch transaction type constants
  • Merge and registration type constants

• Add ProtocolConstantsTest.java with tests for:

  • Magic code bytes
  • Version constants
  • Frame length and head length
  • Message type constants
  • Configured codec and compressor validation

• Update changes/en-us/2.x.md with changelog entry

• Add contributor to the list

Verifying this change

This change is a test-only change and can be verified by running:
\\�ash
mvn test -pl core -Dtest=LockStatusTest,MessageTypeTest,ProtocolConstantsTest
\\

Checklist

• Make sure that the change is in accordance with the contributing guide
• Add unit tests for the change
• Update the changelog

Closes #6502 (partial)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.14%. Comparing base (340aa3c) to head (d642431).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7901      +/-   ##
============================================
- Coverage     71.20%   71.14%   -0.06%     
  Complexity      797      797              
============================================
  Files          1300     1300              
  Lines         49620    49620              
  Branches       5874     5874              
============================================
- Hits          35331    35304      -27     
- Misses        11371    11401      +30     
+ Partials       2918     2915       -3     

see 8 files with indirect coverage changes

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