Fix ignored number_of_replicas and add a --dns-timeout alias (#889)

Two fixes from the #888 review cycle's flagged items:

- [elasticsearch]/[opensearch] number_of_replicas is no longer ignored
  when number_of_shards is not also set. The parser read replicas only
  inside the shards branch — accidental nesting introduced in the
  6.4.0-era code (1c9a6c4) — while docs/source/usage.md lists the two
  options independently and elastic.py/opensearch.py accept them as
  independent parameters with independent defaults (shards=1,
  replicas=0). Regression tests cover the replicas-only case for both
  sections.

- The CLI accepts --dns-timeout as an alias of --dns_timeout, which is
  kept unchanged for backward compatibility (public since 6.0.0;
  --dns-retries, added in 9.7.1, already hyphenated). The end-to-end
  test exercises both spellings through a real _main() run, so dropping
  either option string fails the suite. usage.md's CLI-help block is
  regenerated to match the new --help output.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/source/usage.md

@@ -41,7 +41,7 @@ options:
   -n NAMESERVERS [NAMESERVERS ...], --nameservers NAMESERVERS [NAMESERVERS ...]
                         nameservers to query: IP addresses, https:// URLs (DNS over HTTPS), and/or
                         tls://ip[:port][#hostname] (DNS over TLS)
-  -t DNS_TIMEOUT, --dns_timeout DNS_TIMEOUT
+  -t DNS_TIMEOUT, --dns_timeout DNS_TIMEOUT, --dns-timeout DNS_TIMEOUT
                         number of seconds to wait for an answer from DNS (default: 2.0)
   --dns-retries DNS_RETRIES
                         number of times to retry DNS queries on timeout or other transient errors (default: 0)