ci: bump actions/cache v4 -> v6 (Node 24 runtime)

Node 20 actions are deprecated on GitHub runners; actions/cache@v4 was
the last remaining action targeting it (MCP server tests job warning).
v6 runs on node24 like the rest of the workflow actions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -114,7 +114,7 @@ jobs:
       # (run 31252869254). Persist the model across runs instead; HF_CACHE_DIR
       # is honored by vector-search-provider.ts.
       - name: Cache embedding model
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: ${{ github.workspace }}/.hf-cache
           key: hf-model-Xenova-all-MiniLM-L6-v2