Add k8s.exec operation for executing commands in containers

[Cali0707]

Description

Add a k8s.exec operation to execute commands inside running containers. This is critical for functional verification testing in tasks like multi-container-pod-communication, statefulset-lifecycle, and setup-dev-cluster.

Current bash tasks use patterns like:

• kubectl exec pod-name -c container -- curl -s localhost:80
• kubectl exec pod-name -n namespace -- cat /data/test
• kubectl exec pod-name -- nslookup kubernetes.default
• kubectl exec pod-name -c logger -- ls -la /var/log/nginx/

Requirements

• Support executing arbitrary commands in a pod's container
• Support specifying target container in multi-container pods
• Support namespace specification
• Capture stdout/stderr and exit code
• Support an expect block for inline verification of output

Example Usage

Check HTTP response:

- k8s.exec:
    pod: communication-pod
    namespace: multi-container-logging
    container: web-server
    command: ["curl", "-s", "-o", "/dev/null", "-w", "%{http_code}", "localhost:80"]
    expect:
      stdout:
        match: "200"

Verify file contents:

- k8s.exec:
    pod: db-0
    namespace: statefulset-test
    command: ["cat", "/data/test"]
    expect:
      stdout:
        equals: "initial_data"

Acceptance Criteria

• Can execute a command in a pod by name and namespace
• Can target a specific container using container name
• Defaults to first container when container not specified
• Supports expect.stdout for output assertions (equals, match, contains)
• Supports expect.exitCode for exit code verification
• Handles pod not found / container not found errors
• Operation is documented in extension manifest