BackedEnum normalization

[andrepimpao]

Summary

• Normalize string- and integer-backed enums in query parameters and headers.
• Support nested query values, header lists, API defaults, endpoint options, and direct Api::send() calls.
• Convert enum-backed header values to strings for PSR-7 compatibility.
• Preserve existing query behavior for null, false, 0, and empty strings.
• Document backed-enum support as available from version 3.1.0.

Reviewer Notes

Normalization occurs after API defaults and endpoint options are merged, providing consistent behavior at the shared transport boundary. A strict request-factory fixture verifies that integer-backed header values do not rely on permissive PSR-7 implementations.