fix(api/pipeline): correct type for path query param + add testing

[ecrupper]

Immediate follow up to #1076, as splitting a query value on a comma is very finicky given URL encoding and fortunately Gin and "github.com/google/go-querystring/query" (sdk-go) both handle multi key queries as a slice input.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.89%. Comparing base (0d08fc3) to head (4204f72).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1100      +/-   ##
==========================================
- Coverage   63.28%   61.89%   -1.40%     
==========================================
  Files         348      358      +10     
  Lines       10603    10885     +282     
==========================================
+ Hits         6710     6737      +27     
- Misses       3407     3662     +255     
  Partials      486      486              

Files	Coverage Δ
api/pipeline/compile.go	55.10% <100.00%> (ø)

... and 9 files with indirect coverage changes

[plyr4]

good call