[jeremydmiller]

Closes #5435

What was wrong

A relational message store's agent Uri is wolverinedb://{engine}/{host}/{database}/{schema}. It leaves out the port: PostgreSQL and MySQL keep the port apart from the host, and SQL Server's host,port is split at the comma. Two tenant databases with the same name on the same host but different ports (two clusters or two containers on one box) therefore shared one Uri. MessageStoreCollection keys stores by Uri, so only the first database got a durability agent. The second database's scheduled messages were never released, and its dead nodes' work was never recovered.

Found by the durability chaos rig, leg 8 (#5414). In a 20 second run with one pause fault, all 820 of the second tenant's delayed messages stayed Scheduled with owner 0.

The fix

• New internal IPortQualifiedAgentUri, implemented by MessageDatabase<T>. It exposes the port and can switch the Uri to wolverinedb://{engine}/{host}:{port}/{database}/{schema}. The qualified Uri is built only when it is asked for.
• MessageStoreCollection.categorizeStore checks for the collision. If a non-main store's Uri is already taken by another database on a different port, each of the two that is not the main store is qualified. The colliding Uri is remembered, so a database discovered later with that Uri is qualified too. Nodes that discover tenant databases in different orders therefore end up with the same Uris.
• The same database reached twice (same port) is not a collision. The main store never changes its Uri.
• MySqlMessageStore.Describe() now sets Port.

No Uri changes for any configuration that works today. Qualification only happens for the collision, so agent assignments, node_assignments rows and CritterWatch's DatabaseUri stay as they are for everyone else. DurabilityProjectionAffinity already normalizes ports away, so projection affinity is unaffected.

Tests

• CoreTests.Persistence.tenant_databases_told_apart_by_port_5435: collision, a later colliding database, collision with the main store, same database twice, and no collision. Without the fix, 3 of 5 fail.
• PostgresqlTests.agent_uri_carries_the_port_when_asked_5435 and SqlServerTests.agent_uri_carries_the_port_when_asked_5435 run offline against the real stores.
• Locally green: CoreTests Persistence + Runtime.Agents (760), PostgresqlTests MultiTenancy (29), MartenTests MultiTenancy (79), PersistenceTests ModularMonoliths (28).
• Rig leg 8 smoke (seed 80): before, 820 delayed messages lost. After, CLEAN.

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