fix: minimized warnings generated by macro expanded code

[soumyamahunt]

addressed warnings for:

• unused container variables with nested decoding
• no mutation to encode container variable for enums with internal tagging
• unused encode container variable for enum cases without any associated variables to be encoded
• availability warning for extensions of type that has @available attribute
• unreachable catch clause for untagged enums with fallback case
• handling as expression with CodedAs with community help: https://forums.swift.org/t/discrepancy-between-swift-ast-explorer-and-swift-syntax/75103/3

[codecov]

Codecov Report

Attention: Patch coverage is 92.95775% with 10 lines in your changes missing coverage. Please review.

Project coverage is 94.73%. Comparing base (3d5be63) to head (2d40924).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...Variables/Enum/Switcher/UnTaggedEnumSwitcher.swift	85.91%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
- Coverage   94.87%   94.73%   -0.15%     
==========================================
  Files         109      109              
  Lines        3202     3302     +100     
==========================================
+ Hits         3038     3128      +90     
- Misses        164      174      +10     

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[soumyamahunt]

One warning which results with macro @CodedAs(nil as String?) usage handling not working as expected and being discussed: https://forums.swift.org/t/discrepancy-between-swift-ast-explorer-and-swift-syntax/75103

[UPDATE]
Discussion completed with resolution.