validate: Recognize Kubernetes built-in resources

[ihopenre-eng]

Description of your changes

crossplane resource validate currently reports built-in Kubernetes resources as missing schemas because it only looks for matching CRD/XRD schemas.

This change falls back to the canonical client-go Kubernetes scheme when custom schema lookup misses. Recognized built-in GVKs are treated as valid, while unknown custom GVKs continue to report missingSchema. Regression coverage includes both a core v1/Secret and an apps/v1/Deployment.

Fixes #195

Validation

• go test ./pkg/validate -count=1
• All cmd/crossplane/validate tests except the upstream Windows-only TestLocalCacheExists/DoesNotExist fixture
• go vet ./pkg/validate
• golangci-lint run --no-config ./pkg/validate/... (0 issues)
• git diff --check

TestLocalCacheExists/DoesNotExist fails on Windows because its expected path uses /; the same failure reproduces at clean upstream commit 5342ad7. Nix is unavailable locally, and the repository's pinned golangci-lint 2.11.4 rejects the current .golangci.yml because goconst.ignore-tests is no longer supported.

I have:

• Read and followed Crossplane's contribution process.
• Run ./nix.sh flake check to ensure this PR is ready for review.
• Added or updated unit tests.
• Linked a PR or a docs tracking issue to document this change.
• Added backport release-x.y labels to auto-backport this PR.

Summary by CodeRabbit

• Bug Fixes
  • Built-in Kubernetes resources are now recognized as valid even when no custom schema is available.
  • Resources without schemas continue to be reported as missing schemas when they are not recognized by Kubernetes.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9f87fceb-4ce0-4927-9aab-abc3d377c761

📥 Commits

Reviewing files that changed from the base of the PR and between 5342ad7 and b1a3de7.

📒 Files selected for processing (2)

• pkg/validate/validate.go
• pkg/validate/validate_test.go

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📝 Walkthrough

Walkthrough

Built-in Kubernetes resources recognized by the client scheme now validate as valid without CRD schemas. Tests cover core Secret and apps/v1 Deployment resources.

Changes

Built-in resource validation

Layer / File(s)	Summary
Recognize built-in resource kinds pkg/validate/validate.go	Validation uses the Kubernetes client scheme to mark recognized GVKs as valid and retains missing-schema handling for unrecognized kinds.
Validate built-in resource fixtures pkg/validate/validate_test.go	Adds Secret and Deployment fixtures and verifies valid aggregate and per-resource results.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: jcogilvie

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: recognizing Kubernetes built-in resources during validation.
Linked Issues check	✅ Passed	The change addresses #195 by treating recognized built-in resources like Secret and Deployment as valid instead of missing schema.
Out of Scope Changes check	✅ Passed	The diff is tightly scoped to built-in resource recognition and regression tests, with no obvious unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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