Simplify implementation for forming input arguments

[knoepfel]

This PR simplifies input argument handling by removing the JoinNodePort template parameter from the product retriever. The retrieve() method now takes a single message instead of the full messages collection, eliminating unnecessary template complexity.

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PR summary credit: Claude Sonnet 4.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

❌ Your project status has failed because the head coverage (76.49%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   76.39%   76.49%   +0.09%     
==========================================
  Files         124      124              
  Lines        2733     2731       -2     
  Branches      475      475              
==========================================
+ Hits         2088     2089       +1     
+ Misses        449      447       -2     
+ Partials      196      195       -1     

Flag	Coverage Δ
unittests	76.49% <100.00%> (+0.09%)	⬆️

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

Files with missing lines	Coverage Δ
phlex/core/declared_fold.hpp	94.82% <100.00%> (ø)
phlex/core/declared_observer.hpp	100.00% <100.00%> (ø)
phlex/core/declared_predicate.hpp	89.65% <100.00%> (ø)
phlex/core/declared_transform.hpp	91.48% <100.00%> (ø)
phlex/core/declared_unfold.hpp	94.73% <100.00%> (ø)
phlex/core/input_arguments.hpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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