Fix tests for notifications (#446)

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## Motivation and Context
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## How Has This Been Tested?
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## Breaking Changes
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## Types of changes
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- [x] Bug fix (non-breaking change which fixes an issue)
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- [ ] I have read the [MCP
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kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/FeatureNotificationService.kt

@@ -348,7 +348,7 @@ internal class FeatureNotificationService(
         job.start()
     }
 
-    /** Returns the current timestamp in milliseconds. */
+    /** Returns the current timestamp in milliseconds since Unix epoch. */
     @OptIn(ExperimentalTime::class)
     private fun getCurrentTimestamp(): Long = clock.now().toEpochMilliseconds()