Make proxy cache vote deduplication atomic

[BenCodez]

Summary

• synchronize server-cache vote insertion across the duplicate check, in-memory add, and persistence
• synchronize online-cache vote insertion across the same operation
• add concurrent regression tests for both cache paths

Why

The existing contains-then-add flow was correct for sequential delivery but was not atomic. Two simultaneous deliveries with the same voteId could both pass the duplicate check and be cached/persisted twice.

Impact

Duplicate reservation is now atomic per VoteCacheHandler instance. The synchronized sections are short and vote traffic is low-volume, so contention should be negligible.

Validation

The new tests start two delivery threads behind a latch and assert that only one cache entry and one persistence call are produced for server and online caches.