Spike: add test coverage for event-key dedup edge cases

[OmarAlJarrah]

Context

#132 added the de-duplication that keeps the reserved event key single-valued when the event(name) tag collides with a global-context, MDC, or per-event-field key. Two branches are handled but not yet exercised by tests:

• Clear-after-set: event("").field("event", "x") — an empty name clears the tag, so the user's field("event") should pass through as an ordinary field, with no suppression and no diagnostic.
• Cross-logger throttle isolation: the once-per-logger diagnostic guard lives on the ClientLogger instance, so two distinct loggers should each emit the diagnostic once. Only the single-logger throttle is currently covered.

Scope

Add the two regression tests in LoggingEventTest. Small, but they pin behaviour that is easy to regress in a future refactor of the dedup or throttle paths.