Handle null link-to events from deleted/scavenged streams

[condron]

Closes #189

Summary

When EventStore resolves link events against deleted or scavenged streams, the resolved Event property is null while the link event retains its position. Stream positions are immutable, so downstream checkpoints remain valid.

Changes

Null event guards (EventStoreConnectionWrapper.cs)

• ToRecordedEvents: skips null entries when converting ResolvedEvent[] to RecordedEvent[]
• Subscription callbacks (SubscribeToStream, SubscribeToStreamFrom, SubscribeToAll, SubscribeToAllFrom): check evt.Event != null before invoking eventAppeared

Deserialize guard (JsonMessageSerializer.cs)

• Returns null for events with null/empty Metadata or Data instead of throwing during JObject.Parse

TcpBus port-collision fix

• Replaced hardcoded port 10008 with dynamic GetFreePort() in TcpBusClientSideTests and TcpBusServerSideTests to prevent failures when net8.0 and net10.0 tests run in parallel

Tests

• when_handling_null_link_to_events: 5 tests covering null-skipping and event number preservation in ToRecordedEvents
• when_deserializing_null_or_empty_event_data: 5 tests covering null/empty metadata and data in Deserialize

Documentation

• Inline XML doc comments on ToRecordedEvents and Deserialize
• Standalone doc: Docs/null-linkto-handling.md

Code formatting

• dotnet format applied solution-wide (separate commit)

[joshkempner]

Let's drop the reformatting commit and do that in a separate PR with an editorconfig file that we agree on.