What this PR does / why we need it:

Adds client-side form validation and fixes entity/data source population in the Control Plane UI create/edit flows.

DataSourceFormModal

• Add missing validation for Redshift (database + table required), Spark (table or path required), and all batch sources (timestamp field required for point-in-time correctness)
• Add format validation for File URI (scheme check) and Kafka bootstrap servers (host:port format)
• Clear source-type-specific errors when source type changes
• Add isSubmitting / submitError props so backend errors appear inside the modal instead of a dismissing page banner

FeatureViewFormModal

• Fix broken imports: replace non-existent useLoadEntitiesREST / useLoadDataSourcesREST with useResourceQuery
• Populate Entities combo-box and Batch Source dropdown from live REST API data on open
• Fix entity/data source name resolution to use spec.name (REST API shape)
• Show actionable empty-state warnings with inline Create buttons when no entities or data sources exist

Calling pages (data-sources/Index, DataSourceOverviewTab, feature-views/Index, RegularFeatureViewOverviewTab, entities/EntityOverviewTab)

• Pass isSubmitting={applyXxx.isLoading} — Submit button shows a spinner while request is in-flight
• Pass submitError={errorMessage} — errors shown inside the open modal
• Clear error state on modal close to prevent stale errors on re-open

Which issue(s) this PR fixes:

#6482

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change