fix: apply defaults to absent exploded query arrays

[Arvin-Lee]

Fixes #1114.

Exploded query arrays previously ignored their declared default when the parameter was absent, leaving the slice nil. Apply the comma-separated default only when the query parameter is missing, preserving explicit values and non-exploded behavior.

Adds seven regression cases covering missing parameters, explicit single and repeated values, literal commas, and empty values.

Validation on Go 1.27.1:

• All new regression cases pass; missing-parameter cases failed before the fix.
• Full tests and race tests pass except for four existing YAML tests, which also fail on unmodified HEAD.
• Examples compile using a temporary module file to resolve existing missing go.sum entries.
• git diff --check passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.19%. Comparing base (8e4bada) to head (1e4b8dd).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1116      +/-   ##
==========================================
+ Coverage   93.16%   93.19%   +0.02%     
==========================================
  Files          24       24              
  Lines        4991     5008      +17     
==========================================
+ Hits         4650     4667      +17     
  Misses        273      273              
  Partials       68       68              

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[wolveix]

Thanks @Arvin-Lee!