feat: add Alexa to PlatformEnum

[chrismParticle]

Summary

Consumers need to send data associated with Alexa devices through the Java SDK. The SDK must recognize Alexa as a supported device platform to construct and serialize those events, so ALEXA is required in DeviceInformation.PlatformEnum. This follows the precedent established by the Samsung TV and Smart TV enum additions.

• add Alexa to DeviceInformation.PlatformEnum using the server contract value alexa
• add focused enum round-trip and model assignment coverage matching the existing platform test pattern
• document the addition in the Unreleased changelog

Testing

• ./gradlew test (8 tests passed)
• sent a development test event locally and verified that the Alexa platform data appeared correctly in Live Stream

[chrismParticle]

Test successful

[cursor]

PR Summary

Low Risk
Low risk enum addition with accompanying unit test coverage and no changes to existing logic.

Overview
Adds ALEXA ("alexa") to DeviceInformation.PlatformEnum to allow events to be tagged with Alexa device platforms. Also introduces unit test coverage verifying enum serialization and model assignment.

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