Add provider-backed refund reservation contention tests

[teesofttech]

Problem

The SQLite unit test validates in-process coordination but cannot prove serializable contention behavior across separate application instances for every production provider.

Scope

• Run concurrent reservations through independent connections/process-equivalent contexts
• Cover SQL Server, PostgreSQL, and MySQL where CI infrastructure permits
• Assert accepted pending/refunded totals never exceed captured amount
• Exercise retrying execution strategies and deadlock/serialization retries

Raised from PR #114 review.