Fix OutboxJoin schema initialization and InMemoryLease concurrency issues in fanout tests

[Copilot]

FanoutCoordinatorIntegrationTests failed with "Invalid object name 'dbo.OutboxJoin'" and had concurrency issues in the test lease implementation.

Changes

• Schema initialization: Added missing EnsureOutboxJoinSchemaAsync call to create OutboxJoin/OutboxJoinMember tables
• Lease acquisition race condition: Replaced check-then-act with atomic AddOrUpdate pattern
• FencingToken thread safety: Changed to Interlocked operations for increment/read
• Background task lifecycle: Tracked expiration task and properly awaited during disposal
• Test data consistency: Fixed workKey mismatch ("daily" → "default") in cursor advancement test

// Before: race condition between check and insert
while (leases.TryGetValue(resourceName, out var existing)) {
    if (existing.IsExpired) leases.TryRemove(resourceName, out _);
    else return null;
}
leases[resourceName] = lease;  // Can overwrite another thread's lease

// After: atomic operation
var resultingLease = leases.AddOrUpdate(
    resourceName,
    _ => newLease,
    (_, existing) => existing.IsExpired ? newLease : existing);

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid clearing active lease when acquisition is blocked

When a non-expired lease already exists, AddOrUpdate returns it and we immediately dispose the freshly created newLease (lines 310-313). DisposeAsync always invokes RemoveLease, so this path removes the currently held lease from the dictionary even though we return null, letting a subsequent caller acquire a second lease while the original holder is still active. Any concurrent acquisition attempt against an active lease can therefore break exclusivity and allow multiple coordinators to run simultaneously.

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