ci: add runtime-examples job for the smol and async-std examples

Bring back the e2e smoke test for the alternative runtime examples
that test_suite.sh used to run. Putting it in its own job, with a
service-container broker, instead of re-entangling it with the
integration suite:

- the integration test job uses testcontainers and a random host
  port, so it can't host examples that hardcode localhost:9092;
- the examples are runtime-correctness checks for smol and async-std,
  not broker-compatibility checks, so a single pinned Kafka version is
  enough and matrixing across 3.7-4.0 would be wasted CI time.

The service container is apache/kafka:4.0.2 in KRaft mode with the
same single-broker overrides we set in tests/utils/containers.rs
(transaction-state-log RF/ISR = 1), exposed on localhost:9092 with a
kafka-topics.sh-based healthcheck so the runner waits for the broker
before launching cargo. The examples produce / consume one message
each and exit non-zero on failure.

Verified locally that both examples compile with
`--no-default-features --features cmake-build`. The check job's
cargo build --all-targets continues to catch pure compile breakage;
this job catches anything that only manifests at runtime.

Author: j7nw4r

.github/workflows/ci.yml

@@ -94,3 +94,42 @@ jobs:
           KAFKA_VERSION: ${{ matrix.kafka-version }}
           RUST_LOG: off
           RUST_BACKTRACE: 1
+
+  # Smoke-test the smol and async-std runtime examples against a real
+  # broker. The integration suite covers the tokio path via testcontainers;
+  # this job catches breakage in the alternative runtimes that
+  # `cargo build --all-targets` would miss.
+  runtime-examples:
+    runs-on: ubuntu-24.04
+    services:
+      kafka:
+        image: apache/kafka:4.0.2
+        ports:
+          - 9092:9092
+        env:
+          KAFKA_NODE_ID: 1
+          KAFKA_PROCESS_ROLES: broker,controller
+          KAFKA_LISTENERS: PLAINTEXT://:9092,CONTROLLER://:9093
+          KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://localhost:9092
+          KAFKA_CONTROLLER_LISTENER_NAMES: CONTROLLER
+          KAFKA_CONTROLLER_QUORUM_VOTERS: 1@localhost:9093
+          KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: CONTROLLER:PLAINTEXT,PLAINTEXT:PLAINTEXT
+          KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
+          KAFKA_TRANSACTION_STATE_LOG_REPLICATION_FACTOR: 1
+          KAFKA_TRANSACTION_STATE_LOG_MIN_ISR: 1
+        options: >-
+          --health-cmd "/opt/kafka/bin/kafka-topics.sh --bootstrap-server localhost:9092 --list"
+          --health-interval 5s
+          --health-timeout 5s
+          --health-retries 12
+          --health-start-period 30s
+    steps:
+      - uses: actions/checkout@v4
+      - uses: lukka/get-cmake@latest
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: ${{ env.rust_version }}
+      - run: sudo apt-get update
+      - run: sudo apt-get install -y libcurl4-openssl-dev
+      - run: cargo run --example runtime_smol --no-default-features --features cmake-build -- --topic smol
+      - run: cargo run --example runtime_async_std --no-default-features --features cmake-build -- --topic async-std