fix(dns_czechia): read _normalizeJson input from stdin, not as an argument (#7077)

_normalizeJson reads its JSON from stdin (sed | sed | tr) and ignores
any positional argument. dns_czechia_add() called it as
`_normalizeJson "$_res"`, so the response was discarded and the inner
sed blocked reading from stdin.

When issuing for a single domain, or for a record that already exists,
the "already exists" branch returns early and never reaches this call,
which is why the bug stayed hidden. With multiple domains, the first
record often short-circuits on "already exists" while the next,
freshly-added record reaches the broken call and hangs on interactive
runs (or consumes unrelated stdin non-interactively).

Pipe the response into _normalizeJson via stdin, matching
dns_czechia_rm() and every other dnsapi plugin.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

dnsapi/dns_czechia.sh

@@ -76,7 +76,7 @@ dns_czechia_add() {
     return 0
   fi
 
-  _nres="$(_normalizeJson "$_res")"
+  _nres="$(printf '%s' "$_res" | _normalizeJson)"
   if [ "$?" -ne 0 ] || [ -z "$_nres" ]; then
     _nres="$_res"
   fi