[OFREP] Add SSE event stream support

[jonathannorris]

Context

ADR-0008 defines Server-Sent Events (SSE) as an optional real-time change notification mechanism for OFREP. The OpenAPI spec PR adds the corresponding eventStreams field to evaluation responses.

The OFREP provider currently does stateless per-request evaluation with no change detection. SSE support would allow the provider to receive real-time notifications when the underlying flag configuration changes and emit PROVIDER_CONFIGURATION_CHANGED events so the host application can react.

Scope

Parse the optional eventStreams array from the POST /ofrep/v1/evaluate/flags/{key} response and connect to any entries with type: "sse". On receiving a refetchEvaluation event, emit a PROVIDER_CONFIGURATION_CHANGED event.

1. Parse eventStreams and manage SSE connections (High)

• On a successful evaluation response, check for an eventStreams array
• For each entry with type: "sse", resolve the connection URL from either url or endpoint.origin + endpoint.requestUri
• Ignore entries with unknown types for forward compatibility
• Maintain a single SSE connection (deduplicate across evaluation responses returning the same URL)

2. Handle change events (High)

• On receiving an event with data.type: "refetchEvaluation", emit PROVIDER_CONFIGURATION_CHANGED
• On SSE connection failure, continue reconnection attempts

3. Inactivity timeout (Low)

• Apply an effective inactivityDelaySec: client override > response value > default 120 seconds
• On timeout, close the connection; on resume, reconnect

References

• ADR-0008
• OpenAPI spec changes
• launchdarkly/dotnet-eventsource — .NET SSE client library