fix(apiform): support primitive pointers in comma arrays

[sylvesterkaczmarek]

Summary

Dereference non-nil pointer/interface elements when encoding multipart arrays with FormatComma, while preserving the existing empty-string representation for nil elements.

Problem

internal/apiform.encodeArray already special-cases nil pointer/interface elements in comma-form arrays, but non-nil pointers are never dereferenced before the primitive-type switch.

As a result, a valid primitive array such as:

[]*string{&first, nil, &second}

fails with:

comma format not supported for complex array elements

even though the pointed-to values are ordinary strings and the same encoder supports primitive values directly.

Fix

Unwrap pointer and interface layers before primitive comma serialization. If a nil value is encountered at any layer, keep the existing behavior of adding an empty comma element.

Complex non-primitive elements continue to return the same error as before.

Regression coverage

Added a focused multipart regression for []*string{&first, nil, &second} and verify it serializes as:

alpha,,beta

Validation

The branch is based directly on current upstream main (ee92673a416c0851a5fe8907a2453db7bd450633) and contains one commit touching only internal/apiform/encoder.go plus focused regression coverage. Full Go test execution is left to repository CI.

Risk

Low. The change only affects pointer/interface elements inside FormatComma arrays. Direct primitives, nil elements, complex-element rejection, and all other multipart array formats retain their existing behavior.