fix: resolve silent request skipping in PRIORITY scheduling

[Tejas-Raj01]

Purpose

Fixes #49097.

This PR resolves a logic bug in the SchedulingPolicy.PRIORITY preemption path. When a request is preempted, the scheduler's request bookkeeping (req_index) was not correctly adjusted if the preempted request appeared earlier in the self.running list than the current iteration cursor. This caused the subsequent request to be silently skipped for the entire scheduling step.

Test Plan

1. Created a regression test in tests/v1/core/test_priority_preemption_bug.py that simulates the request-skipping scenario under sustained KV pressure.
2. Verified the fix by running:
   pytest tests/v1/core/test_priority_preemption_bug.py -v
3. Ran local scheduler tests to ensure no regressions:
   pytest tests/v1/core/test_scheduler.py -v

Test Result

Before Fix:
Regression test failed:
AssertionError: Bug present: Request C was silently skipped...

After Fix:
Regression test passes:
tests/v1/core/test_priority_preemption_bug.py PASSED

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[claude]

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[github-actions]

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[Tejas-Raj01]

@WoosukKwon @robertgshaw2-redhat @njhill @ywang96 @alexm-redhat @heheda12345 @ApostaC @orozery @ivanium

ready for review.

[njhill]

Thanks @Tejas-Raj01, just requesting a couple of minor adjustments.

[Tejas-Raj01]

Hi @njhill, thanks for the review on PR #49206. I have applied your suggested adjustments and rebased the branch onto the latest main. Could you please take a look when you have a moment? It might need the ready label again to trigger the CI. Thanks!

[github-actions]

✅ @Tejas-Raj01, CI is now available for this PR.

• /ci run starts a CI build.
• /ci retry retries failed jobs in the CI build for the current PR head. If the current head has no CI build, it starts a new CI build for the current head containing only jobs that failed in the latest earlier CI build for this PR.

[Tejas-Raj01]

/ci run

[njhill]

/ci run

[github-actions]

✅ Triggered Buildkite CI #82707 for commit fbc42290812d.