Fix Perceived Streaming On Proxied Hosts (Pantheon or similar)

Tested this MR on Drupal 10 with DDEV (nginx + PHP-FPM 8.3) against an AI Assistant routed through /agui/api/chat. Confirmed the patch behaves as designed:

Server-side wiring

• StreamedResponse headers correctly include Content-Type: text/event-stream, Cache-Control: no-cache, Connection: keep-alive, and X-Accel-Buffering: no.
• Transfer-Encoding: chunked (no Content-Length), TTFB ~57ms with content download spanning the full 11s of the run — true incremental streaming.

Padding / delay knobs

• Default initialPaddingSize=4096 produces the leading : SSE-comment with 4096 spaces.
• Default chunkPaddingSize=1024 produces a : comment between every data: event, visible in the raw response body.
• Setting chunkPadding=0 (via query string or POST body) removes the inter-event padding lines.
• Setting chunkDelay=500 introduces a clearly visible per-chunk pause; the assistant message paints token-by-token with the configured delay.

Abuse caps from the follow-up commit

• chunkDelay=99999 is correctly clamped to 1000ms server-side.
• chunkPadding=99999 is correctly clamped to 8192 bytes server-side.
• Negative values clamp to 0.

Both transports

• Confirmed the params are accepted from both the JSON request body ({"chunkDelay":..., "chunkPadding":...}) and the URL query string (?chunkDelay=...&chunkPadding=...).

Caveat

• The original buffering symptom only reproduces behind a proxy (Fastly/Pantheon). On local DDEV streaming works regardless of the new settings, so I could only validate the plumbing locally. The padding + delay overrides are exactly the levers needed to defeat Fastly's buffering on Pantheon, and the implementation is sound.

RTBC from my perspective.