C# client SDK: remove use of ?connection_id query param to subscribe route

[gefjon]

Currently, the SDK generates its ConnectionId locally, and passes it to the subscribe HTTP route. This is wrong; the server should (and would, given the opportunity) generate a unique ConnectionId for the connection and return it to the client. Rework the C# client SDK to that effect.

DoD: When running a C# client, the host logs do not contain any lines like:

2025-02-19T15:53:04.862398Z DEBUG /home/phoebe/clockworklabs/SpacetimeDB/crates/client-api/src/routes/subscribe.rs:74: The connection_id query parameter to the subscribe HTTP endpoint is internal and will be removed in a future version of SpacetimeDB.