This PR refines SSE server generation to format event data more accurately and efficiently.

• Add to SSE data to determine when to build the event payload from the generated HTTP response body type.
• Use the generated response body () for SSE payloads when present, ensuring parity with normal HTTP responses.
• Encode primitives directly (string/bytes/bool/numbers), fall back to only for complex values.
• Expose to the SSE template funcs.
• Update SSE golden files accordingly.

Rationale:

• Aligns SSE data shape with existing response body converters, avoiding divergence between streaming and non-streaming endpoints.
• Avoids unnecessary JSON for primitives, improving readability and performance for clients.

Tests:

• ok goa.design/goa/v3/http/codegen 1.145s passes locally; goldens updated.