Add unit tests for applierGroup.Get in Kubernetes executor

Description

Currently, there is no unit test coverage for the routing logic in the applierGroup.Get method. The production code has a pending comment requesting testing coverage for this component. This issue proposes adding unit tests to verify that the Kubernetes applier is correctly resolved and returned based on matching routes (Kind, Name, Labels, and fallback).

Relevant Code

• pkg/app/piped/executor/kubernetes/applier_group.go

Proposed Changes

• Implement TestApplierGroupGet in applier_group_test.go.
• Cover the following scenarios using mocked appliers:
  • Fallback to defaultApplier when routes are empty or matchers don't align.
  • Successful routing by provider name.
  • Multi-applier construction when routing by provider labels.
  • Error propagation when matched providers or appliers are missing.

Start with pkg/app/piped/executor/kubernetes/applier_group.go and inspect the existing applier_group_test.go before running its package tests. Implement TestApplierGroupGet with mocked appliers covering fallback, provider-name routing, provider-label multi-applier construction, and missing-provider or applier errors. Done means the routing cases and error propagation are covered by passing unit tests.