Improve E2E Test Coverage for Webhooks (Defaulting, Validation, Conversion) and Cert-Manager CA Injection

[camilamacedo86]

What do you want to happen?

The current e2e tests for webhooks in the Kubebuilder repository cover basic webhook creation and validation. However, several important scenarios remain untested. To ensure comprehensive coverage, we propose expanding the tests to include:

• Defaulting Webhooks: Ensure defaulting webhooks apply resource defaults as expected.
• Validation Webhooks: Confirm that invalid resources are rejected by validation webhooks and validation logic is enforced.
• Conversion Webhooks: Verify conversion webhooks work when converting resources between different versions of CRDs.
• Cert-Manager CA Injection: Ensure cert-manager correctly injects CA certificates into all webhook types (defaulting, validation, and conversion), and verify that these webhooks function properly in a cluster.

Context:
There are existing tests for webhooks, such as in:

• plugin_cluster_test.go
• generate_test.go
• webhooks.go

These tests can be added under a new directory in e2e tests or, if more convenient, integrated into the existing go/v4 test suite.

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