feat: add automatic health check wait option to ContainerGuard

[joshrotenberg]

Summary

Add a builder option to automatically wait for the container to be ready before returning from start().

Current API

let guard = ContainerGuard::new(template).start().await?;
guard.template().wait_for_ready().await?;  // separate call required

Proposed API

let guard = ContainerGuard::new(template)
    .auto_wait_ready(true)  // or .wait_on_start(true)
    .start()
    .await?;
// Container is guaranteed ready at this point

Benefits

• Single call for "give me a ready container"
• Reduces boilerplate in tests
• Can still disable for cases where you don't need to wait

Considerations

• Should this be the default behavior? (breaking change if so)
• Timeout configuration for the wait
• Error handling if readiness check fails

Related to #218 (wait_for_ready convenience method)

Part of testing utilities enhancements.

[joshrotenberg]

This was already implemented in PR #225 (Issue #218).

The API is:

let guard = ContainerGuard::new(RedisTemplate::new("test"))
    .wait_for_ready(true)
    .start()
    .await?;
// Container is guaranteed ready at this point

The wait_for_ready(true) builder option automatically waits for the container to pass its readiness check before start() returns.