[nathanwhit]

Browsers can send split Cookie fields over HTTP/2. Deno already joined cookies with "; " for bulk request-header exposure and the raw backend, but the hyper-backed single-header getter used the generic ", " separator. That made req.headers.get("cookie") disagree with iteration and broke cookie parsing for frameworks that read cookies through get().

This change centralizes request-header value separator selection, applies it to the hyper single-header getter, and keeps the raw and bulk paths on the same rule. It also adds raw HTTP/1 and raw HTTP/2 tests that send physically split Cookie and generic X-Test fields, then assert both headers.get() and header iteration produce consistent combined values.

Validation:

deno run --allow-read --allow-write --allow-run --allow-net ./tools/format.js
cargo build -p deno --bin deno
./target/debug/deno test --config tests/config/deno.json --no-lock --allow-all --location=http://js-unit-tests/foo/bar --filter httpServerHttp1SplitHeaderConcatenation tests/unit/serve_test.ts
./target/debug/deno test --config tests/config/deno.json --no-lock --allow-all --location=http://js-unit-tests/foo/bar --filter httpServerHttp2SplitHeaderConcatenation tests/unit/serve_test.ts