fix(admission): validate subgroup names are lowercase on PodGroup creation

[cluster2600]

Closes #948.

Problem

The scheduler internally lowercases subgroup parent names via formatParentName() in pkg/scheduler/api/podgroup_info/subgroup_info/factory.go, but this constraint was never validated at admission time. A user creating a PodGroup with mixed-case subgroup names (e.g. Workers) would have their resources accepted by the API but then silently hit parent-lookup failures during scheduling, which is quite confusing to diagnose.

Solution

Added lowercase validation in the PodGroup validating webhook (validateSubGroups) that rejects:

• Subgroup names containing uppercase characters
• Parent references containing uppercase characters

Both ValidateCreate and ValidateUpdate paths are covered since they share the same validation function. This ensures the constraint is properly recognised and enforced before the resource reaches the scheduler.

Changes

• pkg/apis/scheduling/v2alpha2/podgroup_webhook.go — added strings.ToLower checks for SubGroup.Name and SubGroup.Parent in validateSubGroups()
• pkg/apis/scheduling/v2alpha2/podgroup_webhook_test.go — updated existing test fixtures to use lowercase names (matching the new constraint), added three new test cases:
  • Uppercase subgroup name rejected
  • Mixed-case subgroup name rejceted
  • Uppercase parent reference rejected

Test plan

• All existing TestValidateSubGroups cases pass with lowercase fixture names
• New validation cases cover uppercase name, mixed-case name, and uppercase parent

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[cluster2600]

Closing — upstream merged #1383 which adds kubebuilder validation patterns (^[a-z0-9]([-a-z0-9]*[a-z0-9])?$) on both Name and Parent fields in the CRD spec. This enforces lowercase at the API level, making the webhook-level validation here redundant. Cheers to @itsomri for landing it!