feat: add ContainerGuardSet for multi-container test management

[joshrotenberg]

Summary

Implements Issue #220 - ContainerGuardSet for managing multiple containers as a group with coordinated lifecycle.

Features

• Type-erased storage for heterogeneous template types (Redis, Postgres, etc. in the same set)
• Shared network support with automatic creation/cleanup
• Coordinated startup via start_all()
• Access containers by name via contains() and names()
• Automatic cleanup on drop with panic handling (keep_on_panic)
• Builder pattern for configuration

API

use docker_wrapper::testing::ContainerGuardSet;
use docker_wrapper::RedisTemplate;

#[tokio::test]
async fn test_with_multiple_containers() -> Result<(), Box<dyn std::error::Error>> {
    let guards = ContainerGuardSet::new()
        .with_network("test-network")
        .add(RedisTemplate::new("redis-1"))
        .add(RedisTemplate::new("redis-2"))
        .keep_on_panic(true)
        .start_all()
        .await?;

    assert!(guards.contains("redis-1"));
    assert!(guards.contains("redis-2"));
    assert_eq!(guards.network(), Some("test-network"));

    // All containers cleaned up on drop
    Ok(())
}

Implementation Details

• Uses trait objects (Box<dyn PendingEntryTrait>) for type erasure
• Stores cleanup functions as boxed closures for Drop implementation
• Network creation/cleanup coordinated with container lifecycle
• Sequential startup ensures predictable ordering

Tests

Added 6 new integration tests:

• test_container_guard_set_basic - two container lifecycle
• test_container_guard_set_with_shared_network - network sharing
• test_container_guard_set_empty - edge case handling
• test_container_guard_set_single_container - single container case
• test_container_guard_set_network_no_auto_remove - network persistence
• test_container_guard_set_wait_for_ready_disabled - readiness config

Closes #220