Add explicit Apply method to ClientApplicator

[DMilmont]

When consumers use a newer controller-runtime than the SDK, the promoted method set on ClientApplicator breaks. This will also be a problem if/when the SDK itself upgrades.

Note: The SDK itself is not yet on controller-runtime v0.22, making this a preemptive fix. That said, the change is minimal and removes complexity that any consumer on v0.22+ would otherwise have to work around independently. It also means one (minor) fewer thing to address if/when this is upgraded to v0.22+

This is probably worth discussing if this is worth adding now vs later (v0.22+)

Closes # #114

💸 TL;DR

controller-runtime v0.22 added Apply to client.Client. Since ClientApplicator embeds both client.Client and Applicator (which also has Apply), Go's method promotion becomes ambiguous for consumers on v0.22+. This adds an explicit Apply method that delegates to Applicator.Apply, resolving the ambiguity with zero behavioral change.

📜 Details

Background:

• PR Fix controller-runtime v0.23 Apply ambiguity #108 fixed internal SDK call sites affected by this ambiguity but did not address the ClientApplicator struct's public API.
• PR add option to do server side apply #113 (server-side apply option) is orthogonal and can coexist with this change.
• This fix is backwards compatible: on controller-runtime v0.21 (current), promotion still works the same way. The explicit method simply makes the delegation unambiguous regardless of controller-runtime version.

What changed:

• Added func (ca *ClientApplicator) Apply(...) that delegates to ca.Applicator.Apply(...)
• Added var _ Applicator = (*ClientApplicator)(nil) compile-time check

Why now:
Downstream consumers that depend on achilles-sdk but use controller-runtime v0.22+ are currently broken by this ambiguity. This will also be required when achilles-sdk itself upgrades to v0.22.

🧪 Testing Steps / Validation

• go build ./... passes across the entire SDK
• Compile-time interface satisfaction check (var _ Applicator = (*ClientApplicator)(nil)) serves as a regression guard
• Existing tests in pkg/io/applicator_test.go exercise applicator.Apply(...) and compile cleanly with the new explicit method
• Zero behavioral change - the explicit method delegates identically to what promotion previously resolved

✅ Checks

• CI tests (if present) are passing
• Adheres to code style for repo
• Contributor License Agreement (CLA) completed if not a Reddit employee - am Reddit employee

[erik-ringsmuth]

I like this approach! I'm adding @harveyxia as well since he had a different idea, but I think this is better because it's not a breaking change.

[harveyxia]

Awesome, nice and simple fix that is backwards compatible.

And users will still have access to the controller-runtime method by calling c.Client.Apply()