docs-gates: close HTTPError response; name failed assertion in smoke logs (CR round 2)

Two accepted GitHub-side CodeRabbit findings on the smoke gate:

Close the HTTPError response stream: _probe_once read the non-2xx body via
e.read() but never closed the HTTPError, which is file-like and owns the
response socket — leaking it / raising ResourceWarning on the expected-404 path
every run. The read is now wrapped in "with e:" INSIDE the crash-containment
nesting, so the stream is closed even if the read raises (still yielding the
retryable transport-error result, never a traceback).

Name the failed assertion in retry/fail logs: _probe_once now returns a compact
`detail` naming which check failed ("status" / "docs-build" / "apex-build" /
"search-mount", multiple joined by "+", or the transport error text; None when
ok) instead of a bare transport-only field. SMOKE-RETRY / SMOKE-FAIL lines gain
`detail=<...>` alongside the existing status / docs-build fields, so an apex-build
or search-mount failure no longer logs an opaque "status=200 docs-build=<sha>".
ok-path behavior and the JSON / exit contract are unchanged.

Tests: HTTPError stream is closed on the happy-404 path and when the read raises
(RecordingBody close recorder); apex-build-only and search-mount-only failures
name their detail in the logs; _probe_once joins multiple failed checks with "+".
Removed the now-unused _http_error helper.

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