Remove System.exit(1) from async samples

[Copilot]

Addresses feedback from #47952: System.exit(1) in async samples terminates the JVM, breaking any application that embeds or copies this code.

Changes

Removed System.exit(1) from error handlers in three async samples:

• Sample06_GetAnalyzerAsync.java - Uses doFinally() for cleanup
• Sample15_GrantCopyAuthAsync.java - Uses explicit latch.countDown() in handlers
• Sample16_CreateAnalyzerWithLabelsAsync.java - Uses explicit latch.countDown() in handlers

Pattern

Before:

.subscribe(
    result -> { /* success */ },
    error -> {
        error.printStackTrace();
        System.exit(1);  // Kills JVM
    }
);

After (doFinally pattern):

.doFinally(signalType -> latch.countDown())
.subscribe(
    result -> { /* success */ },
    error -> { /* error logged in doOnError */ }
);

After (explicit pattern):

.subscribe(
    result -> { latch.countDown(); },
    error -> { latch.countDown(); }
);

Both patterns allow errors to propagate naturally without killing the process.

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