fix(h2): forward 1xx informational responses to onInfo

[pacocartones]

What

onInfo is documented as "Invoked for each informational (1xx) response. May be invoked multiple times." HTTP/1 forwards them — client-h1.js onHeadersComplete forwards a statusCode < 200 via onResponseStart. HTTP/2 does not: it only subscribes to the stream response event (the final response) and never to the headers event, where Node emits informational headers. So 1xx responses (102 Processing, 103 Early Hints) are silently dropped on h2 and onInfo never fires.

Why it matters

103 Early Hints is increasingly used for preload/preconnect, and it is exactly the kind of thing an h2/CDN path serves. Any client.request({ onInfo }) over an ALPN-negotiated h2 connection loses them today, diverging from h1 with no documented reason.

The fix

Subscribe to the stream headers event and forward via onResponseStart, matching the h1 path and reusing the same state == null / aborted || completed guards as onResponse. Only the :status pseudo-header is stripped; 1xx responses carry no other response pseudo-headers.

   stream.on('response', onResponse)
+  stream.on('headers', onInterimResponse)
   stream.on('end', onEnd)

(plus the matching off in the listener teardown, and the onInterimResponse handler).

Test

Adds test/http2-informational.js: an h2 server emits a 103 Early Hints via additionalHeaders before the 200, and the test asserts onInfo receives it with its link header. It fails on main (infos.length 0 vs 1) and passes with the change.

Reachability is plain: allowH2: true (or an Agent negotiating h2) against any server that sends 1xx. No custom store or exotic config.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.47%. Comparing base (5033aa1) to head (2b7f612).

Files with missing lines	Patch %	Lines
lib/dispatcher/client-h2.js	84.61%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5712   +/-   ##
=======================================
  Coverage   93.47%   93.47%           
=======================================
  Files         110      110           
  Lines       38906    38932   +26     
=======================================
+ Hits        36366    36391   +25     
- Misses       2540     2541    +1     

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