fix(docker): install saml extra in litellm-backend image (#39291)

The monolithic images install the saml extra but the split backend image
did not, so /sso/saml/* returned 501 on Helm split-image deployments.
The gateway image is unchanged since /sso/ routes are backend-only.

backend/Dockerfile

@@ -46,6 +46,7 @@ RUN --mount=type=cache,target=/root/.cache/uv \
         --extra proxy-runtime \
         --extra extra_proxy \
         --extra semantic-router \
+        --extra saml \
         --python python3.13
 
 # Stage 2 — copy source and install the project + workspace members.
@@ -57,6 +58,7 @@ RUN --mount=type=cache,target=/root/.cache/uv \
         --extra proxy-runtime \
         --extra extra_proxy \
         --extra semantic-router \
+        --extra saml \
         --python python3.13
 
 RUN HOME=/opt/prisma XDG_CACHE_HOME=/opt/prisma/.cache PRISMA_BINARY_CACHE_DIR=/opt/prisma/binaries \