Consolidate duplicate test logger implementations in logging specs

[Aaronontheweb]

Summary

There are three separate test logger implementations in src/Akka.Hosting.Tests/Logging/ that share significant overlap:

1. TestLogger (TestLogger.cs) — captures messages by log level
2. SemanticTestLogger (SemanticLoggingSpecs.cs) — captures message + structured state dictionary
3. BugReproTestSink (Issue701SemanticLoggingRegressionSpecs.cs) — captures message + structured state dictionary

Implementations 2 and 3 are ~95% identical:

• Both implement ILogger
• Both capture the formatted message and enumerate IEnumerable<KeyValuePair<string, object>> state into a dictionary
• Both write to ITestOutputHelper
• Only minor naming differences (Entries vs LogEntries, BugReproLogEntry vs LogEntry)

Proposed Change

Extract a shared StructuredTestLogger and CapturedLogEntry into a common file (e.g., SharedLoggingTestUtilities.cs) and have both test classes use it. This reduces maintenance burden when the logging infrastructure changes.