Doh: fix and improvements

[icing]

DoH improvements

• decode results when individual requests are done
• makes happy eyeballing start asap
• remove doh_resp structures as no longer needed
• remove CURL_DNS_TYPE_NS, CURL_DNS_TYPE_CNAME and CURL_DNS_TYPE_DNAME from DoH
• DoH: do not set PIPEWAIT and SSL OPTS when url starts with http:
• mark Doh master handle as dirty after every sub-request, not only the last
• Doh: start probe on AAAA before A (was the other way).

cf-dns: set EXPIRE_HAPPY_EYEBALLS timer when waiting 50ms on AAAA result or progress will not be triggered in time.

Add debug env var CURL_DBG_HE_AAAA_AWAIT_MS to override the default 50ms on waiting for the AAAA result to arrive.

test2100: set CURL_DBG_HE_AAAA_AWAIT_MS to 60 seconds to provide enough time for slow CI runs to sent all DoH requests.

[icing]

Ok, the single remaining failure is a Valgrind CI job test2100, that does DoH with A+AAAA queries, but since DoH now supports eyeballing intermediate results, the A answer is acted upon without even enough time to properly start the AAAA query (50ms is the "delay").

That's my current analysis. Not sure how we'd want to stabilize that. Maybe a new env var, like CURL_DEBUG_H3_EYEBALLING_AAAA_WAIT_MS_WHATEVER_ITS_THE_WEEKEND, or so.

[Copilot]

Pull request overview

Improves DoH response handling and Happy Eyeballs timing while simplifying DoH data structures.

Changes:

• Decodes each DoH response as it completes.
• Adds configurable AAAA waiting and timer handling.
• Expands DoH integration tests and server readiness checks.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
lib/vdns/doh.c	Reworks DoH probes, decoding, and result handling.
lib/vdns/doh.h	Simplifies DoH types and response state.
lib/vdns/cf-dns.c	Adds AAAA waiting configuration and timer.
docs/libcurl/libcurl-env-dbg.md	Documents the debug timing variable.
tests/unit/unit1650.c	Updates DNS decoder expectations.
tests/http/testenv/dnsd.py	Adds HTTP-based server readiness checks.
tests/http/test_21_resolve.py	Extends partial-response and HTTP-failure coverage.
tests/data/test2100	Increases AAAA waiting for slow test runs.

Suppressed comments (1)

lib/vdns/doh.c:1122

• Errors from HTTPS RR decoding, including CURLE_OUT_OF_MEMORY, are likewise reclassified as an HTTP failure and ultimately returned as a name-resolution failure. Preserve this CURLcode in the async result and propagate it from Curl_doh_take_result() instead of losing the actual local error.

    if(result) {
      dohp->probe_rc[slot] = DOH_HTTP_FAILED;
      infof(doh, "[DoH] error decoding HTTPS RR: %s",
            curl_easy_strerror(result));
      goto out;

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