fix: register admission webhook handlers

[Al-assad]

Motivation

With controller-runtime v0.20, NewWebhookManagedBy(mgr).For(obj).Complete() only records the API type. It does not automatically register custom defaulters or validators. As a result, enabling ENABLE_WEBHOOK=true can leave the webhook server without registered handlers, and admission calls may fail or be ignored depending on failurePolicy.

The validation methods also need to validate the decoded admission object passed by controller-runtime instead of the receiver used as the validator instance.

Changes

• Explicitly register DorisCluster and DorisDisaggregatedCluster defaulters and validators with WithDefaulter and WithValidator.
• Validate the decoded obj / newObj passed to webhook callbacks.
• Update webhook tests to exercise the same validator-instance shape used by admission handling.

Tests

• go test ./api/doris/v1 ./api/disaggregated/v1
• Verified in a test environment with yulinying/doris-operator:dev-20260707:
  • invalid DDC feSpec.replicas < feSpec.electionNumber is rejected by vdorisdisaggregatedcluster.kb.io
  • invalid DDC adminUser.name=admin is rejected by vdorisdisaggregatedcluster.kb.io
  • valid DDC dry-run is allowed