[http-client-java] Generate polymorphic SSE event models

[XiaofeiCao]

Clear and concise description of the problem

The Java emitter needs a consistent model shape for SSE operations that can emit multiple service-defined event kinds.

Acceptance criteria

• Selects and generates one representation, such as per-event wrappers, a marker hierarchy, or a service union.
• Preserves discriminator information for known event kinds.
• Defines behavior for unknown and terminal event kinds.
• Maintains forward-compatible deserialization and includes generated tests.

Dependencies

• Parent: [FEATURE REQ] Full support for SSE Azure/azure-sdk-for-java#50264
• Public event representation contract: [FEATURE REQ] Define the public server-sent event representation Azure/azure-sdk-for-java#50415

Checklist

• Follow our Code of Conduct
• Read the docs
• Checked for an existing duplicate