feat: add configurable stop timeout for ContainerGuard cleanup

[joshrotenberg]

Summary

Adds configurable timeout for stop operations during ContainerGuard cleanup/drop.

Closes #221

Changes

• Add stop_timeout field to GuardOptions (Option<Duration>)
• Add stop_timeout() builder method to ContainerGuardBuilder
• Use timeout in Drop implementation for both runtime contexts

API

use docker_wrapper::testing::ContainerGuard;
use docker_wrapper::RedisTemplate;
use std::time::Duration;

// Fast cleanup with 1 second timeout
let guard = ContainerGuard::new(RedisTemplate::new("redis"))
    .stop_timeout(Duration::from_secs(1))
    .start()
    .await?;

// Immediate SIGKILL with zero timeout
let guard = ContainerGuard::new(RedisTemplate::new("redis"))
    .stop_timeout(Duration::ZERO)
    .start()
    .await?;

Use Cases

• Fast test cleanup with short timeout for containers that don't need graceful shutdown
• Longer timeout for containers that need time to flush data
• Zero timeout for immediate SIGKILL

Tests

Added 2 integration tests:

• test_container_guard_stop_timeout - 1 second timeout
• test_container_guard_stop_timeout_zero - immediate SIGKILL