fix: CI failures — add protoc, fix TLS crypto provider in test

- Add protobuf-compiler install step to Check & Test CI job
  (warpgrid-raft uses tonic-build which requires protoc)
- Fix tls_cert_chain_validation test: install ring crypto provider
  before rustls operations (CryptoProvider::install_default)
- Add Default impl for MockBackend (missed in previous clippy fix)

Verified locally: cargo fmt, clippy -D warnings, and cargo test --all
all pass with zero failures.

Author: nadilas

.github/workflows/ci.yml

@@ -16,6 +16,8 @@ jobs:
     runs-on: blacksmith-4vcpu-ubuntu-2404
     steps:
       - uses: actions/checkout@v4
+      - name: Install protoc
+        run: sudo apt-get update && sudo apt-get install -y protobuf-compiler
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy, rustfmt

crates/warpgrid-cluster/tests/integration_cluster.rs

@@ -259,6 +259,9 @@ async fn concurrent_join_safety() {
 
 #[tokio::test]
 async fn tls_cert_chain_validation() {
+    // Install the ring crypto provider for rustls.
+    let _ = rustls::crypto::ring::default_provider().install_default();
+
     // Generate a CA (verifying the public API works).
     let (ca_pair, _ca_cert) = tls::generate_ca().unwrap();
     assert!(ca_pair.cert_pem.contains("BEGIN CERTIFICATE"));