wire-api: strip framing headers in generic streaming response renderers

[jschaul]

A follow-up to #5381 in other places streaming is done.

== WARNING: AI written. Needs a bit more human review first, keeping this in draft for later... ===

Both generic streaming renderers re-frame the body via Warp (chunked):

• MultiVerb's IsWaiBody (SourceIO ByteString) instance
• LowLevelStream's servant HasServer route

Each spliced the response's own headers into the Wai.responseStream
verbatim. If a Content-Length (or Transfer-Encoding) rode along, Warp
would honour it and serve the streamed body under that declared length
instead of chunking; any mismatch between the declared length and the bytes
actually streamed then desynchronises the caller's keep-alive HTTP/1.1
connection (the client reads past the response boundary into the next
response). This is the same defect fixed in
Federator.Response.streamingResponseToWai.

No endpoint currently attaches such a header to a streaming response (the
federator's inward path already strips to Content-Type only, and cargohold's
RespondStreaming carries no length), so this is defense-in-depth: strip
Content-Length/Transfer-Encoding in both renderers via a stripFramingHeaders
helper so Warp always frames exactly what is streamed.

Adds Test.Wire.API.Routes.Streaming covering both renderers (verified
red without the strip, green with it).

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines