feat: add with_network() support to ContainerGuard

[joshrotenberg]

Summary

Implements #219. Adds network support to ContainerGuard for multi-container test scenarios.

New Builder Methods

with_network(name)

Attach the container to a Docker network:

let guard = ContainerGuard::new(RedisTemplate::new("redis"))
    .with_network("test-network")
    .start()
    .await?;

create_network(bool)

Control automatic network creation (default: true):

.with_network("existing-network")
.create_network(false)  // Don't create if missing

remove_network_on_drop(bool)

Clean up the network when the guard drops (default: false):

.with_network("test-network")
.remove_network_on_drop(true)  // Clean up network too

network() getter

Get the configured network name:

assert_eq!(guard.network(), Some("test-network"));

Behavior

• Network is created with bridge driver if it doesn't exist (and create_network is true)
• Network cleanup only happens if the guard created the network
• Network is removed after the container is removed (network must be empty)
• Template's network config is automatically updated

Testing

• Added 2 new integration tests
• Updated unit tests for new options
• All 762 existing tests continue to pass

Closes #219