fix: catch FormatException in log formatters to prevent third-party logger crashes

[Aaronontheweb]

Summary

• Catch FormatException in SemanticLogMessageFormatter and DefaultLogMessageFormatter positional format paths, returning a diagnostic string ([INVALID LOG FORMAT] str=[...], args=[...]) instead of throwing
• Add defense-in-depth FormatException catches in LogFilterEvaluator.ShouldTryKeepMessage() to protect against custom formatters that may throw
• Add 8 new unit tests covering both formatters and both evaluator paths

Problem

Bad format strings (e.g., "{0} {1} {2}" with only 2 args) caused FormatException to propagate through ShouldTryKeepMessage() and crash third-party logging actors (Serilog, NLog, etc.). The existing catch in StandardOutLogger.PrintLogEvent() only protected the console logger.

Stack trace from customer report:

System.FormatException: Index (zero based) must be greater than or equal to zero and less than the size of the argument list.
   at Akka.Event.SemanticLogMessageFormatter.Format(String format, IEnumerable`1 args)
   at Akka.Event.LogEvent.ToString()
   at Akka.Logger.Serilog.SerilogLogger.<.ctor>b__11_1(Warning w)

Test plan

• SemanticLogMessageFormatter returns diagnostic for mismatched positional args (both object[] and IReadOnlyList paths)
• DefaultLogMessageFormatter returns diagnostic for mismatched positional args
• LogFilterEvaluator (empty and content-filter paths) handles FormatException gracefully
• Valid format strings still format correctly (regression tests)
• Existing LoggerSpec tests updated and passing

[Arkatufus]

LGTM