vector: bound request buffering before authentication and document installation (#12537)

* s3vectors: bound body buffering before authentication

* docs: include vector engine installation and source builds

* s3vectors: prevent MIME sniffing of JSON error responses

* docs: load vector Cargo configuration when building from source

* docs: note the native protoc requirement for Windows vector builds

* docs: make vector source checkout and working directory explicit

* ci: handle deb822 sources during Ubuntu mirror failover

* ci: bound Ubuntu mirror attempts and reject partial index updates

.github/workflows/e2e.yml

@@ -61,6 +61,9 @@ jobs:
         restore-keys: |
           ${{ runner.os }}-buildx-e2e-
 
+    - name: Test APT mirror fallback
+      run: python3 test_apt_install.py
+
     - name: Install dependencies
       run: |
         sudo rm -f /etc/apt/sources.list.d/azure-cli.list /etc/apt/sources.list.d/microsoft-prod.list