fix(controller/test): use manager's client to access cache with e.g. custom fields

[vdice]

Use mgr.GetClient() instead of the direct client to the raw API server, so that custom fields (i.e. fields specific to the Spin-Operator, e.g. 'spec.executor') are valid.

This fix was provoked by a PR in the queue (#481) in which tests were added making API calls with the spec.executor custom field. Without this fix, errors like the following were occurring due to the raw K8s API server rejecting the field:

2026-03-30T10:08:41-06:00	ERROR	Unable to list SpinApps	{"controller": "spinappexecutor", "controllerGroup": "core.spinkube.dev", "controllerKind": "SpinAppExecutor", "SpinAppExecutor": {"name":"containerd-shim-spin","namespace":"test-ns-a"}, "namespace": "test-ns-a", "name": "containerd-shim-spin", "reconcileID": "fed21f07-b128-4f50-b421-4d2e2ed799b3", "error": "field label not supported: spec.executor"}

[Copilot]

Pull request overview

Updates the SpinAppExecutor controller envtest setup so the reconciler uses the controller-runtime manager’s cached client, enabling MatchingFields queries on custom fields like spec.executor during tests (mirroring production behavior).

Changes:

• Switch SpinAppExecutorReconciler.Client in tests from the direct API client to mgr.GetClient() (cache-backed).
• Ensures field-indexed lookups (e.g., spec.executor) don’t hit the raw API server field-selector validation path.

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[kate-goldenring]

LGTM