feat: delimited multi-value query params — QueryParameters.split and @Query(delimiter)

[OmarAlJarrah]

Summary

Adds first-class support for delimited multi-value query parameters on both sides of the library — reading a delimited value apart in the core, and writing one from an annotated field in the binder. The two halves are inverses of each other.

1. Core — QueryParameters.split (read side)

QueryParameters.parse splits the raw query only on &, so a single pair whose value packs several items behind an in-value delimiter is opaque: for ?roles=admin,user the caller gets back the whole "admin,user" string. Repeated pairs (x=1&x=2) already flatten via getAll, but in-value delimiters (,, |, ;, …) do not.

public fun split(name: String, delimiter: Char): List<String>

Reads every pair for name in appearance order and splits each decoded value on delimiter, so repeated pairs and in-value delimiters both flatten into one list.

val q = Url.parseOrThrow("https://h/?roles=admin,user&perm=read|write&id=1,2&id=3").queryParameters
q.split("roles", ',')                  // ["admin", "user"]
q.split("perm", '|')                   // ["read", "write"]
q.split("id", ',')                     // ["1", "2", "3"]   — pairs and delimiters both flatten
q.split("id", ',').map(String::toInt)  // [1, 2, 3]         — conversion is just map()

Splitting is derived on demand (the snapshot is never mutated and round-trips unchanged), literal, and lossless — no trimming, no empty-token dropping (a,,b → ["a", "", "b"], x= → [""], trailing empties kept). It operates on the decoded value, so a percent-encoded delimiter (a%2Cb) still splits. There is no typed-conversion layer: splitting cannot fail, so callers convert with a plain map in whatever error style they prefer.

2. Binder — @Query(delimiter) (write side)

An iterable @Query field always fanned out into repeated params (roles=admin&roles=user), with no way to emit the single delimited value many APIs expect (?fields=id,name,email; OpenAPI style: form, explode: false).

class Search(
    @Query("roles", ',') val roles: List<Role>,   // -> roles=admin,user
    @Query("tags")       val tags: List<String>,  // -> tags=a&tags=b   (unchanged)
)

When the delimiter is set on an iterable field, the non-null elements' scalar text joins on the delimiter into one parameter value. The default (a NUL sentinel, `'