[FEATURE REQ] Define the public server-sent event representation

**Is your feature request related to a problem? Please describe.**
Java SDKs need a consistent decision on whether SSE operations expose protocol metadata or only service event models. This is a child of #50264.

**Describe the solution you'd like**
Choose between `ServerSentEvent` and direct service event models, and define the public contract.

Acceptance criteria:
- Defines whether `id`, `event`, `data`, comments, and retry values are exposed.
- Distinguishes a value container from parser defaults, validation, reconnection, and retry policy.
- Includes API documentation and contract tests for the selected representation.

**Describe alternatives you've considered**
#50081 proposes `ServerSentEvent`. Search can instead expose its generated event model directly when protocol metadata is not actionable.

**Additional context**
Parent: #50264
Proposal: #50081

**Information Checklist**
- [x] Description Added
- [x] Expected solution specified