Ensure OutboxJoin schema provisioning in fanout integration tests

[Copilot]

The FanoutCoordinatorIntegrationTests.InitializeAsync method was provisioning the fanout schema but not creating the OutboxJoin/OutboxJoinMember tables before instantiating SqlOutboxJoinStore. This caused FanoutSlices_CanJoinDownstreamMessagesIdempotently to fail with "Invalid object name 'dbo.OutboxJoin'" when attempting database operations.

Changes:

• Added DatabaseSchemaManager.EnsureOutboxJoinSchemaAsync call in InitializeAsync before constructing SqlOutboxJoinStore
• Fixed InMemoryLease concurrency issues with proper atomic operations and expiration handling

await DatabaseSchemaManager.EnsureFanoutSchemaAsync(
    ConnectionString,
    fanoutOptions.SchemaName,
    fanoutOptions.PolicyTableName,
    fanoutOptions.CursorTableName).ConfigureAwait(false);

await DatabaseSchemaManager.EnsureOutboxJoinSchemaAsync(
    ConnectionString,
    outboxOptions.SchemaName).ConfigureAwait(false);

joinStore = new SqlOutboxJoinStore(
    Options.Create(outboxOptions),
    NullLogger<SqlOutboxJoinStore>.Instance);

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