fix(prefix-cache): race conditions in indexer lock management

[hexfusion]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

Fixes race conditions in the prefix cache indexer where Add() and RemovePod() release and re-acquire lock mid-operation, allowing stale entries in hashToPods. See linked issue for full details.

Which issue(s) this PR fixes:

Fixes #2500

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @hexfusion. Thanks for your PR.

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[ahg-g]

/ok-to-test
/lgtm
/approve

[ahg-g]

Thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, hexfusion

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Details

Needs approval from an approver in each of these files:

• pkg/epp/framework/plugins/scheduling/scorer/prefix/OWNERS [ahg-g]

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