Adds support for slice query parameters using ParamWrapper types

[sharonjl]

Adds support for slice query parameters using ParamWrapper types (e.g., []OptionalParam[int]). Previously,
parseSliceInto only handled primitive slice element kinds. Struct elements implementing ParamWrapper were
returned as errUnparsable.

Changes

• Added parseArrStructElement helper that unwraps each slice element's ParamWrapper.Receiver() and
  delegates per-element parsing to parseInto, supporting all receiver types including time.Time, url.URL,
  and TextUnmarshaler
• Added case reflect.Struct to parseSliceInto switch for ParamWrapper-implementing struct elements
• Threaded ctx and paramFieldInfo through parseSliceInto so the struct case can call parseInto directly
• ParamReactor.OnParamSet is called per element when implemented, with the unwrapped parsed value matching
  scalar behavior

I am a little unsure of the reflect unwrapping but there doesn't seem to be another approach other than implementing TextMarshal/Unmarshal to support slice query params of type struct.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.06%. Comparing base (1d5dddf) to head (558f466).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
huma.go	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #998      +/-   ##
==========================================
- Coverage   93.13%   93.06%   -0.07%     
==========================================
  Files          23       23              
  Lines        4777     4801      +24     
==========================================
+ Hits         4449     4468      +19     
- Misses        269      272       +3     
- Partials       59       61       +2     

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