Skip kubernetes envtest when assets are missing

[rootp1]

What this PR does:

Adds an envtest asset preflight to TestExecutor_ensureSync so direct package tests skip with a clear message when etcd or kube-apiserver are unavailable, and adds focused unit coverage for the preflight helper.

Why we need it:

Direct go test on the Kubernetes executor package currently fails with an environment/setup error unless contributors use the Makefile wrapper that provisions envtest assets. This change makes that failure mode explicit and less misleading.

Which issue(s) this PR fixes:

Fixes #7013

Does this PR introduce a user-facing change?:

• How are users affected by this change: Not applicable; test-only change for contributors.
• Is this breaking change: No.
• How to migrate (if breaking change): Not applicable.

Summary

• add a preflight check for required envtest binaries before starting envtest.Environment
• skip the envtest-dependent Kubernetes executor test with a clear message when assets are unavailable
• add focused coverage for envtest binary detection

Linked Issue

Fixes #7013

What Changed

• added requireEnvtestAssets and binary detection helpers in pkg/app/piped/executor/kubernetes/sync_test.go
• gated TestExecutor_ensureSync behind the preflight check
• added TestMissingEnvtestBinaries

Verification

• go test ./pkg/app/piped/executor/kubernetes -run 'TestMissingEnvtestBinaries|TestEnsureSync|TestExecutor_ensureSync' -count=1 — passed
• KUBEBUILDER_ASSETS='' go test ./pkg/app/piped/executor/kubernetes -run TestExecutor_ensureSync -count=1 -v — passed
• make check — not run successfully: failed in build/web because yarn is not installed in this environment
• make test/go MODULES=. — failed: upstream test suite currently fails in pkg/app/piped/executor/kubernetes and pkg/app/piped/platformprovider/kubernetes on this checkout
• make check/dco — failed: upstream commit 25ae69937e55e8330bff96c34346718f3e5e7148 is missing a Signed-off-by line, so the repository DCO check does not pass cleanly on top of upstream/master

Scope

• only pkg/app/piped/executor/kubernetes/sync_test.go was changed; unrelated files were not modified