[CI] Build the Rust extension modules once per run instead of in every CUDA job (#33384)

scripts/ci/cuda/ci_install_dependency.sh

@@ -326,6 +326,40 @@ uninstall_stale_flashinfer() {
     mark_step_done "${FUNCNAME[0]}"
 }
 
+require_prebuilt_rust_exts() {
+    # Stages whose download succeeded set this to none. Runs before
+    # setup_pip_toolchain uninstalls sglang, so clearing it here still reaches
+    # install_sglang below - setup.py reads it from the environment at build time.
+    if [ "${SGLANG_BUILD_RUST_EXTS:-}" != "none" ]; then
+        mark_step_done "${FUNCNAME[0]}"
+        return
+    fi
+
+    # Exact EXT_SUFFIX rather than a _core*.so glob: no crate sets abi3, so a module
+    # built for another minor version satisfies the glob while the import system
+    # ignores it, leaving is_rust_server_built() false and the Rust-server tests
+    # silently skipped. Stages have no setup-python, so the interpreter is whatever
+    # the image ships, and the pools are not on one version (h20 is 3.12 while
+    # h100 is 3.10) - a mismatch is drift to route around, not a failure.
+    local suffix
+    suffix=$(python3 -c 'import sysconfig; print(sysconfig.get_config_var("EXT_SUFFIX"))')
+    local missing=()
+    local pkg
+    for pkg in server grpc multimodal; do
+        [ -f "python/sglang/srt/${pkg}/_core${suffix}" ] || missing+=("${pkg}")
+    done
+    if [ ${#missing[@]} -gt 0 ]; then
+        echo "::warning::no prebuilt _core${suffix} for: ${missing[*]}; building from source"
+        ls -l python/sglang/srt/*/_core*.so 2>/dev/null || echo "(no extension modules at all)"
+        export SGLANG_BUILD_RUST_EXTS=
+        mark_step_done "${FUNCNAME[0]}"
+        return
+    fi
+    echo "Using prebuilt Rust extension modules; skipping the cargo build."
+
+    mark_step_done "${FUNCNAME[0]}"
+}
+
 install_sglang() {
     EXTRAS="dev,runai,tracing"
     if [ -n "$OPTIONAL_DEPS" ]; then
@@ -654,6 +688,7 @@ main() {
     install_apt_packages
     clean_site_packages
     setup_cargo_cache
+    require_prebuilt_rust_exts
     setup_pip_toolchain
     remove_stale_cuda12_nvidia_wheels
     uninstall_stale_flashinfer