Add DebuggerTypeProxy for InMemorySink to simplify debugging

[vfofanov]

Improve debug experience for InMemorySink

Summary

This PR improves debugging of InMemorySink in IDE (and other debuggers) by adding a debugger type proxy that shows log events in a readable, expanded form. It also refines InMemorySink internals (cached read-only view of events, nullable, simplified Instance getter) and adds a predicate overload for Snapshot() to create filtered snapshots.

Changes

Debugger type proxy

• [DebuggerTypeProxy(typeof(InMemorySinkAdvDebugProxy))] on InMemorySink so that when you inspect an InMemorySink instance in the debugger, the value is displayed via the proxy instead of the raw type.
• InMemorySinkAdvDebugProxy – wraps the sink and exposes its log events as a List<DebugLogEvent> so the debugger shows a clear, expandable list of entries.
• DebugLogEvent – a debug-time view of a single log event with:
  • Level – LogEventLevel
  • Message – rendered message (logEvent.RenderMessage())
  • MessageTemplate – template string
  • Properties – log event properties
  • Exception – exception if present
  • OriginalLogEvent – underlying LogEvent for deep inspection
  • ToString() – returns the rendered message for quick scanning in the Locals/Watch window

This makes it easier to inspect what was logged without expanding raw LogEvent and Serilog internals.

InMemorySink behavior and API

• Cached read-only view – LogEvents is backed by a cached ReadOnlyCollection<LogEvent> (_logEventsSnapshot) that is invalidated whenever Emit is called, so repeated enumeration of LogEvents does not allocate and stays consistent within a “no new emit” window.
• Snapshot(Func<LogEvent, bool> predicate) – new overload that returns an InMemorySink snapshot containing only log events that match the given predicate (e.g. filter by level, message template, or property).
• Nullable – #nullable enable and use of ReadOnlyCollection<LogEvent>? for the cached snapshot.
• Instance getter – simplified to LocalInstance.Value ??= new InMemorySink().
• Snapshot() – parameterless Snapshot() now builds the snapshot from GetLogEvents().ToList() (using the cached read-only view when valid).

Benefits

• Easier debugging – In the debugger, expanding an InMemorySink shows a list of DebugLogEvent with Level, Message, MessageTemplate, Properties, and Exception, instead of raw collections and Serilog types.
• Filtered snapshots – Snapshot(predicate) supports assertions or inspection over a subset of events (e.g. only errors, or only a given message template) without manual filtering in test code.
• Stable enumeration – Cached read-only snapshot for LogEvents avoids repeated allocation and gives a consistent view when no new events are emitted.

Checklist

• Solution builds with no errors or warnings.
• All tests pass.
• No breaking API changes for existing LogEvents or parameterless Snapshot() usage; new Snapshot(predicate) is additive.