Validate cross-referenced parameters between PostgresCluster and PostgresClusterClass

[limak9182]

Description

• Adds admission-time cross-resource validation for PostgresCluster against its referenced PostgresClusterClass
• Uses mgr.GetAPIReader() (live API read, not cached) to avoid false rejections during cache propagation

Key Changes

Rule	Field	Condition
Class existence	spec.class	Referenced PostgresClusterClass must exist
Storage floor	spec.storage	Cannot be lower than class default
Version floor	spec.postgresVersion	Major version cannot be lower than class default
Pooler enablement	spec.connectionPoolerEnabled	Cannot be true when class has it disabled
Merge completeness	spec.instances, spec.postgresVersion, spec.storage	After overlay, all three must be resolvable from cluster or class

Testing and Verification

• unit test cases with fake client.Reader covering all rules and edge cases
• Existing pg_hba validation tests updated to pass nil reader (no behavioral change)
• go build ./... passes
• go test ./pkg/splunk/enterprise/validation/... passes

Related Issues

Jira tickets, GitHub issues, Support tickets...

PR Checklist

• Code changes adhere to the project's coding standards.
• Relevant unit and integration tests are included.
• Documentation has been updated accordingly.
• All tests pass locally.
• The PR description follows the project's guidelines.

[github-actions]

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