client/v3: stop concurrency waits when session expires

[ayushk-1801]

Description

When a mutex or election contender is waiting behind an older key, waitDeletes only watches the key ahead of it. If the waiting session expires during that time, its own lease-backed key is removed but the operation keeps waiting until the older key is deleted.

This changes the wait logic to also watch the contender's own key and return as soon as that key disappears.

For mutexes this returns the existing ErrSessionExpired. For elections it returns ErrElectionNotLeader.

I also added regression coverage for both cases so the tests verify that the waiter returns while the current lock holder or election leader is still active.

Fixes #19597

Testing

• go test ./client/v3/concurrency -count=1
• go test ./tests/integration/clientv3/concurrency -count=1
• repeated the new mutex and election regression tests 50 times
• ran the relevant tests with the race detector
• make verify

[kubernetes-prow]

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[kubernetes-prow]

Hi @ayushk-1801. Thanks for your PR.

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[ayushk-1801]

@ivanvc Can you take a look at this and share your feedback?