Fix e2e secret reference schema failures

[Copilot]

The e2e-tests job failed because VectorStore fixtures included an unsupported key field in SecretReference, while Credential reconciliation was blocked by an incorrectly required initProvider secret reference.

• VectorStore fixtures
  • Remove key from litellmParamsSecretRef; Crossplane SecretReference supports only name and namespace.
• Credential schema
  • Make the optional init-provider secret reference omittable.
  • Align the generated CRD schema accordingly.

litellmParamsSecretRef:
  name: e2e-vectorstore-values
  namespace: upbound-system

[Breee]

@copilot already dying e2e tests because of panics

[Copilot]

@copilot already dying e2e tests because of panics

Fixed the nil *litellm.Client metadata panic and the generated initProvider secret-reference validation in e9c6f5f.