feat: add wait_for_ready() method and builder option to ContainerGuard

[joshrotenberg]

Summary

Implements #218. Adds convenience method and builder option to ContainerGuard.

Changes

New Methods

wait_for_ready() on ContainerGuard:

let guard = ContainerGuard::new(template).start().await?;
guard.wait_for_ready().await?;  // Convenience method

New Builder Option

wait_for_ready(bool) builder method:

let guard = ContainerGuard::new(template)
    .wait_for_ready(true)  // Automatically wait after start
    .start()
    .await?;
// Container is guaranteed ready at this point

Behavior

• The builder option makes start() block until the container passes its readiness check
• Works for both new and reused containers (when reuse_if_running is enabled)
• Delegates to the underlying template's wait_for_ready() implementation

Testing

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

Closes #218