mod_http2: RST_STREAM incomplete HTTP/2 response to avoid hang

[mmontalbo]

Summary

On 2.4.x, when an HTTP/2 request handler emits a final response (status,
headers, maybe a partial body) and then stops without sending EOS, e.g. a
CGI that hits the server Timeout mid-body, the stream is left open with no
explicit END_STREAM / RST_STREAM. A client without its own stall timeout waits
indefinitely. The same handler over HTTP/1.1 closes the connection, so the
client fails fast.

Bugzilla: https://bz.apache.org/bugzilla/show_bug.cgi?id=70131

Root cause

mod_http2 already intends to reset such a response: stream_data_cb()
(h2_stream.c) sends RST_STREAM when it pulls APR_EOF from output that never
delivered EOS. But reaching that pull depends on c1 re-dispatching
stream_data_cb after the output beam goes terminal, and s_c2_done() does not
force it: h2_beam_is_complete() treats a plainly closed beam as complete (it
keys on beam->closed, not on whether an EOS was seen), so the reset rides on a
c1/c2 wakeup that is racy under concurrency. On trunk the worker appears to use the
generic ap_process_connection() and the truncated response reaches s_c2_done()
incomplete (so it resets) whereas 2.4.x's bespoke worker closes the beam first, so it
looks complete and never resets.

Fix

Make the terminal state observable independent of timing. When c2_process()
finishes with a final response started but no EOS seen, abort the output
beam instead of closing it. An aborted beam returns APR_ECONNABORTED from
h2_beam_receive() on the next pull unconditionally (it is checked before the
transfer loop), so the existing reset fires regardless of how the wakeup is
scheduled.

Header-only responses (204/304, and HEAD) are complete without a body EOS, so
they are marked complete (response_eos_seen) where the response is finalized
in h2_c2_filter.c; they must not be aborted, or a body-less 304 from mod_cache
revalidation would be RST_STREAM'd again, re-opening
PR 69580.

Tests

Adds two regression tests to test_105_timeout.py:

• test_h2_105_20: a CGI that goes silent past Timeout, run as concurrent
  streams, must be reset.
• test_h2_105_21: a mod_cache revalidation 304 must close cleanly (guards the
  PR 69580 regression).