test(examples): guard golden.Previewer type assertions

[sourcehawk]

Description

The resource shape tests across the examples asserted res.(golden.Previewer)
without checking the result. If a resource ever stopped satisfying Previewer,
those tests would panic rather than fail with a readable assertion. The guarded
form is already what docs/getting-started.md and docs/testing.md teach
consumers to write, and the mutation tests sitting in these same files already
guard their concepts.MutationInspector casts, so the examples were
contradicting both the documentation and their own neighbouring code.

Changes

• Replace the unchecked res.(golden.Previewer) assertion with a
  previewer, ok := form plus require.True(t, ok) in the six example resource
  shape tests

Related

Supersedes #158. That PR set out to add controller-level component golden tests,
but that work has since landed on main independently as
examples/*/app/component_test.go with its testdata/ snapshots. Once #158
dropped its now-duplicate controller tests, this cast guarding was the only
change it still carried, so it is being landed here against current main instead
of rebasing a branch whose two commits partly undo each other.

Testing

make all passes clean (exit 0), covering unit tests, lint, formatting, the
scaffold wrapper tests, and go build ./examples/.... The change is confined to
test files; no example or framework behaviour moves. The six affected shape tests
still pass and their golden files are untouched, which confirms the assertions
are running against the same previewers as before rather than being skipped.

[Copilot]

Pull request overview

This PR hardens the example “resource shape” golden snapshot tests by guarding golden.Previewer type assertions, ensuring failures produce readable assertion output instead of panicking. This aligns the examples with the guidance in docs/testing.md and matches the existing guarded-cast style used elsewhere in the examples.

Changes:

• Replace unchecked res.(golden.Previewer) assertions with previewer, ok := ... plus require.True(t, ok) in six example tests.
• Keep golden assertions identical aside from using the guarded previewer variable, so golden behavior/output remains unchanged.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
examples/grace-inconsistency/resources/deployment_test.go	Guard golden.Previewer assertion before calling golden.AssertYAML.
examples/extraction-and-guards/resources/secret_test.go	Guard golden.Previewer assertion before calling golden.AssertYAML.
examples/extraction-and-guards/resources/configmap_test.go	Guard golden.Previewer assertion before calling golden.AssertYAML.
examples/custom-resource/resources/certificate_test.go	Guard golden.Previewer assertion before calling golden.AssertYAML.
examples/component-prerequisites/resources/deployment_test.go	Guard golden.Previewer assertion in table-driven test before calling golden.AssertYAML.
examples/component-prerequisites/resources/configmap_test.go	Guard golden.Previewer assertion before calling golden.AssertYAML.