net.http: support streaming response callbacks and stop limits over HTTP/2

[quaesitor-scientiam]

Summary

The HTTP/2 fetch path added in #27362 buffers the entire response body before
returning. As a result, requests that rely on streaming response behaviour are
not served over HTTP/2 — to avoid silently ignoring them, such requests
currently fall back to HTTP/1.1 (they do not advertise ALPN h2). This issue
tracks adding real streaming support so HTTP/2 reaches parity.

Affected request features (all currently force HTTP/1.1 via
Request.uses_response_streaming()):

• on_progress (RequestProgressFn)
• on_progress_body (RequestProgressBodyFn)
• stop_copying_limit
• stop_receiving_limit

(on_finish already works on the HTTP/2 path.)

What's needed

Feed the per-DATA-frame body chunks from H2Conn's response read loop
(read_response in vlib/net/http/h2_conn.v) into the same callbacks, and
honor the stop limits, matching the HTTP/1.1 semantics in
receive_all_data_from_cb_in_builder:

• call on_progress / on_progress_body per chunk as DATA frames arrive,
• stop_copying_limit: stop appending to the response body after N bytes while
  still firing callbacks and reading to completion,
• stop_receiving_limit: stop reading the stream early.

Likely shape: thread a generic chunk hook + the two limits through
H2ClientRequest (keeping H2Conn decoupled from net.http.Request), and have
the shim (h2_do in backend.c.v) adapt the request's callbacks to it. Then
drop the uses_response_streaming() gate so these requests can negotiate h2.

Acceptance criteria

• on_progress / on_progress_body fire per DATA frame on the HTTP/2 path.
• stop_copying_limit / stop_receiving_limit behave as on HTTP/1.1.
• Streaming requests with enable_http2: true negotiate h2 (gate removed).
• Tests over the in-memory transport assert chunk-by-chunk callback delivery and
  the stop-limit behaviour; a large download_file_with_progress-style case.

References

• net.http: negotiate HTTP/2 in fetch via ALPN (opt-in) #27362 (HTTP/2 fetch shim), Codex review thread on the streaming gap.

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