Implement auto reconnect in the csharp sdk

[aasoni]

Description of Changes

C# SDK: automatic reconnection

Builds reconnection into DbConnection itself, so the connection object, the client cache, table handles and registered callbacks all stay usable across a reconnect and applications manage no replacement objects.

WithAutomaticReconnect() enables this. Losing an established connection fires OnDisconnect, each failed attempt fires OnConnectError, and both now report the number of and delay before the next attempt, or undefined when the SDK will not retry. Retries are unlimited with exponential backoff (1s base, 30s cap, 0.5 jitter), stopping only on an explicit Disconnect(), a failed initial connection, or a reconnect under a different identity.

Each reconnect is a new connection which fires OnConnect again. Live subscriptions are replayed in one SubscribeBatch under fresh query set ids, and the results are reconciled against the stale cache with synthesized deletes, so rows unchanged across the outage produce no callbacks and changed rows produce ordinary update or delete callbacks.

A stable client-generated session_id is sent on every connection so the server can teardown the previous connection if still open (idle timeout didn't fire yet) before accepting a new one for the same session.

The token the server issued is saved and reused, keeping the identity stable for anonymous clients; WithTokenProvider() supplies a fresh token when the saved one is close to expiring or was rejected.

Calls made while disconnected fail fast with DisconnectedError, and calls in flight when the connection dropped settle with UnknownCallResultError, since they may or may not have run.

Behavior is unchanged for connections which do not opt in.

API and ABI breaking changes

No

Rollback safety impact

Requires #5877 to have been deployed in a previous release

Expected complexity level and risk

3 - Change is limited to the typescript SDK and is opt in only (withAutomaticReconnect and withTokenProvider are the new APIs). Change is large but not overly complex.

Testing

Test app that fakes dropping connections and reconnecting and uses spacetime's own token provider to test token with expiry claim in the reconnect scenario as well.