Safer product_query construction

[knoepfel]

Either default-construct product_query or require the constructor with both the product_specification and the layer argument.

Also switch to using default comparisons

[knoepfel]

@wlav, this PR should help avoid the problem you ran into. Alas, the default product_query constructor is required in order for the Boost JSON's product retrieval to work.

This is not an urgent PR.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
phlex/core/product_query.cpp	60.00%	1 Missing and 1 partial ⚠️
phlex/core/message.cpp	0.00%	1 Missing ⚠️

❌ Your project check has failed because the head coverage (76.71%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   76.61%   76.71%   +0.09%     
==========================================
  Files         124      124              
  Lines        2733     2727       -6     
  Branches      475      475              
==========================================
- Hits         2094     2092       -2     
+ Misses        446      442       -4     
  Partials      193      193              

Flag	Coverage Δ
unittests	76.71% <85.71%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
phlex/configuration.cpp	100.00% <100.00%> (ø)
phlex/core/declared_provider.hpp	100.00% <100.00%> (ø)
phlex/core/detail/filter_impl.cpp	95.23% <100.00%> (ø)
phlex/core/edge_creation_policy.cpp	79.06% <100.00%> (ø)
phlex/core/input_arguments.hpp	100.00% <100.00%> (ø)
phlex/core/multiplexer.cpp	79.31% <100.00%> (ø)
phlex/core/product_query.hpp	90.00% <100.00%> (+6.66%)	⬆️
phlex/core/registration_api.hpp	100.00% <100.00%> (ø)
phlex/model/algorithm_name.cpp	51.61% <ø> (-2.68%)	⬇️
phlex/model/algorithm_name.hpp	33.33% <100.00%> (+33.33%)	⬆️
... and 4 more

... and 3 files with indirect coverage changes

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