fix: install libxml2 and libxslt so the saml-auth plugin loads

[shreemaan-abhishek]

What this fixes

The saml-auth plugin added in APISIX 3.17.0 (apache/apisix#13346) pulls in the lua-resty-saml rock. Its native saml.so dynamically links libxml2.so.2 and libxslt.so.1, which are not installed in the runtime images. As a result every worker logs this on startup and the plugin cannot be used:

[error] load_plugin(): failed to load plugin [saml-auth] err: error loading module 'saml'
from file '/usr/local/apisix/deps/lib/lua/5.1/saml.so':
	libxml2.so.2: cannot open shared object file: No such file or directory

Reproduce on the published image:

docker run --rm --entrypoint sh apache/apisix:3.17.0-debian \
  -c "ldd /usr/local/apisix/deps/lib/lua/5.1/saml.so"
# libxml2.so.2 => not found
# libxslt.so.1 => not found

Fix

Install libxml2 + libxslt1.1 (debian/ubuntu) and libxml2 + libxslt (redhat) as runtime dependencies. After the fix, ldd resolves both libraries and APISIX boots with no saml-auth load errors.

Why CI didn't catch it

The Test route step checked compose/apisix_log/error.log, but that file never exists (the container's error.log is symlinked to stderr), so grep ... && exit 1 always short-circuited and startup errors slipped through green. This PR makes the step grep the container logs instead, so a failing plugin load now fails the build.

Verification

Built the patched debian image locally and booted it against etcd:

ldd .../saml.so   # libxml2.so.2 and libxslt.so.1 both resolved
docker logs <apisix>  # 0 "failed to load plugin [saml-auth]" lines

[shreemaan-abhishek]

Upstream packaging fix (the proper long-term fix declaring these as package deps): api7/apisix-build-tools#469. This PR is the docker-image-side mitigation so the published 3.17.0 images work now.