Improve serverless APIs + test coverage

[viseshrp]

Improvements

Add InvalidFieldError for better error reporting.

Add recursive option to as_dict method.

Handle BaseModel instances in get method.

Modify ObjectSet.values_list method.

Always treat GUIDs as strings.

Improve error handling for base save method and template save.

Raise correct exceptions during PNG conversion.

Bypass validation during init when needed for performance.

Disallow invalid kwargs (content, children) in object creation.

Better superclass checks during initialization.

Enhance file and image handling.

Bug Fixes

Fix population of dynamic properties.

Fix get_or_create method (later removed).

Fix Item instantiation errors.

Fix None checks in Validator.

Fix superclass validation logic.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.70732% with 15 lines in your changes missing coverage. Please review.

Project coverage is 75.86%. Comparing base (8252316) to head (91999e7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ansys/dynamicreporting/core/serverless/adr.py	0.00%	8 Missing ⚠️
src/ansys/dynamicreporting/core/serverless/base.py	87.50%	2 Missing and 2 partials ⚠️
...ansys/dynamicreporting/core/serverless/template.py	78.57%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
+ Coverage   74.09%   75.86%   +1.76%     
==========================================
  Files          25       25              
  Lines        7019     7027       +8     
  Branches     1329     1335       +6     
==========================================
+ Hits         5201     5331     +130     
+ Misses       1342     1248      -94     
+ Partials      476      448      -28     

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