treaty2: text/plain streams aren't detected behind HTTP/2 reverse proxies

[NeilTheFisher]

Streaming a text/plain async generator route works on a direct HTTP/1.1 connection but breaks behind any HTTP/2 reverse proxy (Traefik, nginx, Caddy, Cloudflare, etc.). The detection added in #213 requires Transfer-Encoding: chunked, which HTTP/2 doesn't allow as a response header (RFC 7540 §8.1.2.2), so proxies strip it on h1→h2. Eden then falls into await response.text() and the request hangs forever on long-lived streams (while (true) generators stay in pending).

sse() works because the SSE branch matches on Content-Type only.

Repro

new Elysia()
  .get("/live", async function* () {
    let i = 0
    while (true) {
      yield \`\${i}\`
      i++
      await new Promise(r => setTimeout(r, 1000))
    }
  })
  .listen(3000)

• Direct: `curl -i http://localhost:3000/live\` → `Transfer-Encoding: chunked` present, Eden streams.
• Through Traefik on HTTPS/h2: response has no `Transfer-Encoding` header, Eden hangs.

Fix

Drop the `Transfer-Encoding: chunked` clause from the streaming detection in the default branch of the Content-Type switch:

```ts
response.headers.get('content-type')?.startsWith('text/') &&
!response.headers.has('content-length')
```

This does not regress #213 (the issue that originally added the chunked check). Per RFC 7230 §3.3.1, `Transfer-Encoding: chunked` and `Content-Length` are mutually exclusive, so any HTTP/1.1 chunked response already satisfies the remaining `!has('content-length')` check. A non-streaming `text/plain` response continues to be buffered because it carries a `Content-Length`.

Happy to PR with regression tests.

---

Possibly related: #255 / PR #256 fix non-SSE chunk yield semantics (chunks weren't emitting until end). Both fixes are needed for HTTP/2 plain-text streams to work end-to-end.