[Infra] CI: add UI drift guard + regenerate _experimental/out

Adds a CI job that rebuilds the admin UI from source and fails if the
committed static export at litellm/proxy/_experimental/out/ has drifted
from what npm run build produces. This prevents silently shipping stale
UI bytes and is a prerequisite for the non_root Dockerfile streamlining
work, which will stage the UI from _experimental/out/ directly instead
of rebuilding it inside the image.

Also regenerates litellm/proxy/_experimental/out/ to match a fresh
npm run build (Node 20.20.2) — the committed tree had drifted from
source prior to this commit.

Co-authored-by: yuneng-jiang <yuneng-berri@users.noreply.github.com>

.github/workflows/ui-drift-guard.yml

@@ -0,0 +1,47 @@
+name: UI Drift Guard
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    branches:
+      - main
+      - litellm_internal_staging
+      - "litellm_**"
+    paths:
+      - "ui/litellm-dashboard/**"
+      - "litellm/proxy/_experimental/out/**"
+      - ".github/workflows/ui-drift-guard.yml"
+
+jobs:
+  verify-ui-output-fresh:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+        with:
+          persist-credentials: false
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20.20.2"
+          cache: "npm"
+          cache-dependency-path: ui/litellm-dashboard/package-lock.json
+
+      - name: Build UI
+        working-directory: ui/litellm-dashboard
+        run: |
+          npm ci --no-audit --no-fund
+          npm run build
+
+      - name: Compare against committed _experimental/out
+        run: |
+          set -euo pipefail
+          ( cd ui/litellm-dashboard/out && find . -type f -exec sha256sum {} + ) | sort > /tmp/fresh.txt
+          ( cd litellm/proxy/_experimental/out && find . -type f -exec sha256sum {} + ) | sort > /tmp/committed.txt
+          if ! diff -u /tmp/committed.txt /tmp/fresh.txt > /tmp/drift.txt; then
+            echo "::error::UI output is stale. Regenerate litellm/proxy/_experimental/out/ from ui/litellm-dashboard/out/."
+            head -200 /tmp/drift.txt
+            exit 1
+          fi
+          echo "UI output is fresh."