BUG/MEDIUM: http-ana: check the cookie rewrite result before moving the offsets

BUG/MEDIUM: http-ana: check the cookie rewrite result before moving the offsets

In the "rewrite" and "prefix" cookie modes,
http_manage_server_side_cookies() calls http_replace_header_value() and
ignores its return value. When the expansion doesn't fit it returns 0 and
leaves ctx.value untouched, yet the code still computes <delta> from
srv->cklen, advances next and hdr_end by it, and in "prefix" mode writes
"val_beg[srv->cklen] = COOKIE_DELIM" over data that was never moved. The
delimiter thus lands in whatever follows the cookie and the response is
forwarded with that byte corrupted, while the cookie is left unprefixed,
silently losing persistence.

The maxrewrite reserve normally covers the expansion, so this needs the
inserted string to be larger than it. Reproduced both with a 1100-byte
server cookie name and the default tune.maxrewrite, and with a 61-byte one
and "tune.maxrewrite 16": the Set-Cookie comes back untouched and a '~'
appears in the middle of the next header's value.

Let's check the return value like every other rewrite site does: on
failure nothing is touched, the failed_rewrites counters are incremented
and the parsing of that response's cookies stops there.

This has been there since the HTX cookie handling was added in 1.9 by
commit fcda7c685 ("MINOR: proto_htx: Add functions to manage cookies on
HTX messages"). It may be backported to all stable versions.

Reported-by: Claude (ANT-2026-FVC9MZEJ)