Allow WideEventLogger to serve as default package logger

[mishankov]

Summary

• make WideEventLogger implement the package logger interface so it can be used via log.SetDefault(...)
• add Debug/Info/Warn/Error (+ context variants) on WideEventLogger that emit short-lived log.record events
• route simple log calls through the existing sampler path, preserving sampling behavior for both simple logs and explicit wide events
• normalize slog-style args for simple logs (including groups, duplicate keys, and malformed args handling)
• update log package docs to describe using WideEventLogger as the default logger
• add focused tests covering default logger usage, context attribute propagation, sampler behavior, arg normalization, and error attribute handling

Testing

• Added unit tests in log/wideevent_logger_test.go for default logger wiring and simple log behavior
• Added checks for context key extraction (traceId, serviceName, custom context key)
• Added checks that sampler decisions are respected for both Info(...) and WriteEvent(...)
• Added checks for slog arg normalization (slog.Group, duplicate keys, !BADKEY behavior)
• Not run (test execution output was not provided in this patch context)