Make self-assignment safe

[marcosbento]

Description

When implementing the copy assignment operator, it is easy to forget the case of self-assignment.
A simple check for self-assignment can: avoid unnecessary copies, ensure correct management of resources, ensure proper exception handling.

These changes address the clang-tidy diagnostic: bugprone-unhandled-self-assignment.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.83%. Comparing base (3c93fde) to head (655af26).

Files with missing lines	Patch %	Lines
src/eckit/cmd/Arg.cc	0.00%	1 Missing ⚠️
src/eckit/container/KDMapped.cc	0.00%	1 Missing ⚠️
src/eckit/io/EasyCURL.cc	0.00%	1 Missing ⚠️
src/eckit/message/Message.cc	0.00%	1 Missing ⚠️
src/eckit/mpi/Group.cc	0.00%	1 Missing ⚠️
src/eckit/mpi/Status.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #242      +/-   ##
===========================================
+ Coverage    65.81%   65.83%   +0.01%     
===========================================
  Files         1133     1133              
  Lines        57494    57500       +6     
  Branches      4340     4349       +9     
===========================================
+ Hits         37842    37857      +15     
+ Misses       19652    19643       -9     

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[simondsmart]

Trivially correct.