feat(webhook): unblock cluster deletion by removing deletion schedule handling

[jellonek]

Description

This PR unblocks deletion of cluster object by removing the logic around cluster deletion scheduling through annotations.
All other required parts of #1947 (deletion mechanism, tests, handling plugins cleanup) were there already in place.

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation Update
• 🎨 Style
• 🧑‍💻 Code Refactor
• 🔥 Performance Improvements
• ✅ Test
• 🤖 Build
• 🔁 CI
• 📦 Chore (Release)
• ⏩ Revert

Related Tickets & Documents

• Closes [FEAT] - Mechanism to directly delete Clusters #1947

Added tests?

• 👍 yes
• 🙅 no, because they aren't needed
• 🙋 no, because I need help
• Separate ticket for tests # (issue/pr)

Added to documentation?

• 📜 README.md
• 🤝 Documentation pages updated
• 🙅 no documentation needed
• (if applicable) generated OpenAPI docs for CRD changes

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

[Copilot]

Pull request overview

This PR removes the “cluster deletion scheduling via annotations” mechanism (mutating/validating webhook logic, controller handling, and related helpers) so Cluster objects can be deleted directly, and updates docs/e2e accordingly.

Changes:

• Removed deletion-scheduling annotations and reasons (greenhouse.sap/delete-cluster, greenhouse.sap/deletion-schedule, ScheduledDeletionReason) and all associated logic across webhook/controller/util layers.
• Updated e2e and user documentation to off-board clusters via direct deletion instead of scheduling.
• Cleaned up tests/helpers that previously set or validated deletion scheduling.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pkg/lifecycle/reconcile.go	Removes ScheduledDeletionReason constant from lifecycle reasons.
internal/webhook/v1alpha1/cluster_webhook.go	Removes defaulting/validation/delete logic tied to deletion scheduling annotations.
internal/webhook/v1alpha1/cluster_webhook_test.go	Removes deletion-scheduling expectations/cases (but currently leaves failing/invalid test cases; see comments).
internal/test/util.go	Removes helper that annotated clusters for scheduled deletion; deletes clusters directly in tests.
internal/test/assert.go	Stops special-casing Cluster deletion preparation via annotations in generic deletion helper.
internal/controller/plugin/util.go	Removes requeue/condition logic that depended on cluster deletion scheduling.
internal/controller/plugin/pluginpreset_controller.go	Removes filtering/reconciliation tied to scheduled deletion (but currently breaks PluginPreset reconciliation; see comments).
internal/controller/plugin/plugin_controller.go	Removes requeue logic that blocked plugin reconciliation during scheduled deletion.
internal/controller/cluster/status_test.go	Removes test asserting delete condition with ScheduledDeletionReason.
internal/controller/cluster/cluster_status.go	Removes Delete condition handling based on deletion schedule annotations.
internal/controller/cluster/cluster_controller.go	Removes controller-driven deletion based on schedule elapsed time.
internal/clientutil/util.go	Deletes schedule-extraction / deletion-filtering utility functions.
e2e/shared/cluster.go	Off-boarding flow now directly deletes the Cluster resource.
e2e/cluster/expect/expect.go	Removes expectation helper for “deletion is scheduled in ~48h”.
e2e/cluster/e2e_test.go	Removes e2e test that asserted deletion scheduling behavior.
docs/user-guides/cluster/offboarding.md	Rewrites off-boarding docs to direct deletion (has Markdown issues; see comments).
api/well_known.go	Removes well-known constants for deletion scheduling annotations.

Comments suppressed due to low confidence (2)

internal/webhook/v1alpha1/cluster_webhook_test.go:130

• This test case still expects cluster creation to be denied due to a deletion-marker annotation, but the webhook logic that enforced this has been removed. The test should be updated to reflect the new behavior (e.g., allow creation).

		Entry("it should deny creation of cluster with deletion marker annotation",
			test.NewCluster(test.Ctx, "test-cluster", test.TestNamespace,
				test.WithClusterLabel(greenhouseapis.LabelKeyOwnedBy, teamWithSupportGroupName),
			),
			true,
		),

internal/webhook/v1alpha1/cluster_webhook_test.go:207

• ValidateUpdateCluster no longer validates deletion scheduling, but these entries still assert behavior for "valid/invalid deletion schedule" (including expecting an error). This will make the suite fail and should be updated/removed.

		Entry("it should allow update with valid deletion schedule",
			test.NewCluster(test.Ctx, "test-cluster", test.TestNamespace,
				test.WithClusterLabel(greenhouseapis.LabelKeyOwnedBy, teamWithSupportGroupName),
			),
			false,
		),
		Entry("it should deny update with invalid deletion schedule",
			test.NewCluster(test.Ctx, "test-cluster", test.TestNamespace,
				test.WithClusterLabel(greenhouseapis.LabelKeyOwnedBy, teamWithSupportGroupName),
			),
			true,
		),

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