T-1365 Keep the appender running while the JVM shuts down

[PetrHeinz]

The JVM shutdown hook added in #31 calls stop() as soon as the JVM starts shutting down. The JVM runs all shutdown hooks at the same time, so anything the application logs after that moment is rejected by logback ("Attempted to append to non started appender [Logtail]", only visible with a status listener). Frameworks log a lot at exactly that point and stop logback themselves only at the very end: Spring Boot's logging shutdown handler and the Quarkus logback extension's last shutdown task both call LoggerContext.stop(), and so does logback's own <shutdownHook>.

Measured end to end with 0.3.8 against a real source, SIGTERM one second into a four-second request:

• Spring Boot 3.5.16 and 4.1.1 delivered 14 and 13 lines, where 0.3.7 delivered 19 and 18. Missing were the ContextClosedEvent listener's line, "Commencing graceful shutdown", the in-flight request's last line, "Graceful shutdown complete" and the @PreDestroy line, so every rolling deploy loses its shutdown logs.
• Quarkus 3.33 with quarkus-logging-logback lost the ShutdownEvent observer's line.
• A plain app with logback's <shutdownHook> plus a <delay> lost the line its own shutdown hook logged, which 0.3.7 still delivered.

The hook now only sends what is queued, after waiting for a flush in progress, and leaves the appender running. Whatever is logged afterwards is sent when the framework or logback's hook stops logback, and a JVM that exits without anyone stopping logback still gets its queued logs sent as before. The same scenarios with this change: Spring Boot 19 of 19 lines, the logback <delay> case 4 of 4, and return from main, System.exit, SIGTERM and a 25,000-line backlog all still delivered in full.

The first commit only adds testLogsWrittenWhileAFrameworkShutsDownAreSentWhenItStopsLogback and is expected to fail on CI: a child JVM whose own shutdown hook logs a line and then stops logback, like Spring Boot does. The existing exit test moved onto the same helper.

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