[ISSUE #1057] [Java] Enhance MessageImpl to allow subclasses to access properties

[bcaw-ofeer]

Which Issue(s) This PR Fixes

Fixes #1057

Brief Description

1. Change MessageImpl.properties visibility from private to protected
2. Modify getProperties() to return Collections.unmodifiableMap(properties) instead of new HashMap<>(properties)

This change enables:

• Inheritance-based access to properties in subclasses
• Immutable view of properties to prevent external modifications
• Better performance by avoiding unnecessary HashMap allocations
• Maintains backward compatibility while improving extensibility

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.40%. Comparing base (4076e2b) to head (48fd332).
⚠️ Report is 62 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1058      +/-   ##
============================================
+ Coverage     53.26%   53.40%   +0.13%     
- Complexity      651      689      +38     
============================================
  Files           208      211       +3     
  Lines         14303    14038     -265     
  Branches       5845     5532     -313     
============================================
- Hits           7619     7497     -122     
+ Misses         6308     6162     -146     
- Partials        376      379       +3     

Flag	Coverage Δ
java	61.89% <ø> (+0.21%)	⬆️

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