Reorder follower pod validation to surface leader scheduling status

[0xlen]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR improves the diagnostic output of the vpod webhook for JobSets using exclusive placement.

Previously, ValidateCreate checked if a follower pod had a NodeSelector before checking if the leader pod was scheduled. If the leader was stuck in Pending (e.g., due to quotas), the mutating webhook correctly withheld the selector, but the validating webhook immediately rejected the pod with "follower pod node selector not set". This masked the more accurate "expected, transient error" regarding the leader pod's status.

By moving the leader scheduling check first, users will now correctly see the transient error, which guides them to investigate the leader pod's resource constraints rather than assuming a JobSet configuration failure.

Which issue(s) this PR fixes:

Fixes #1187

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

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[k8s-ci-robot]

Hi @0xlen. Thanks for your PR.

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[kannon92]

/ok-to-test

[kannon92]

[AI]
Suggestions

• Minor: The tests only cover error paths. A happy-path test case (follower pod with correct node selector + scheduled leader → no error) would strengthen confidence, but this is a gap in the existing test suite and not something this PR introduced.
• Minor: The test currently doesn't cover the topologyKey missing case (node selector exists but doesn't have the topology key). Again, not a regression

Looks like there are a few missing test cases but I'm happy if you want to address this as a follow up.

[0xlen]

Thanks for pointing this out. I’ve added both the happy-path case and the missing-topologyKey test in this PR:

1. the case where the follower has a nodeSelector but is missing the required topology key
2. the happy path where the leader pod is already scheduled and the follower has the expected topology nodeSelector

[kannon92]

/approve

@GiuseppeTT can you take a look and give your LGTM?

I'm curious if we should consider a cherry pick for this.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 0xlen, kannon92

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[kannon92]

@GiuseppeTT @imreddy13 could one of you take a look here?

[0xlen]

Hi @GiuseppeTT @imreddy13, gentle ping for LGTM when you get a chance. Let me know if cherry pick is required, I can help to create a separate PR. Thanks!