DEV-266 - Return null for discovery mode or multiple hosts in connection string

[w1am]

Changed: Return null for discovery mode or multiple hosts in connection string
Changed: Updated Address field to support null returns
Fixed: #260

This PR refactors the Address property resolution logic in the EventStoreClientConnectivitySettings class. Previously, when a cluster was specified in the connection string, the Address property defaulted to localhost:2113, which was inconsistent behavior. With this change, the logic now returns null when a cluster is specified. New tests have also been added to cover various connection string scenarios.

Changes

Connection string	Behaviour before	Behaviour after
esdb://localhost:1234	localhost:1234	Unchanged
esdb://localhost:1234,localhost:4567	localhost:2113	null
esdb+discover://localhost:1234	localhost:2113	null
esdb+discover://localhost:1234	localhost:2113	null
esdb+discover://localhost:1234,localhost:4567	localhost:2113	null

[linear]

DEV-266 Improve connectivity settings address resolution logic

[josephcummings]

👍