feat(proxy): validate JSON-RPC frames in SSE event stream

[amirejaz]

Background

PR #5288 adds upstream JSON-RPC response validation for the streamable-HTTP transport. SSE is explicitly out of scope because the fix architecture (buffer full body → validate → rewrite or pass through) doesn't work for a streaming event format.

The gap

When an SSE-based MCP upstream sends a structurally invalid JSON-RPC frame (data: line without jsonrpc:"2.0", wrong id type, etc.), the transparent proxy forwards it unchanged. Clients receive the malformed event and may crash, misparse it, or treat it as a legitimate response.

Why it's harder than streamable-HTTP

For streamable-HTTP, ProcessResponse buffers the entire HTTP response body before any bytes reach the client, so it can atomically reject and rewrite to 502.

For SSE:

• The 200 OK and Content-Type: text/event-stream headers are committed before any events arrive. There is no way to rewrite the status code for a malformed event mid-stream.
• A fix needs a per-event streaming interceptor that wraps the SSE body reader, parses each data: line as it arrives, validates the JSON-RPC content, and either synthesizes an error event or closes the stream cleanly on the first invalid frame.
• "Close the stream" is the strongest guarantee available — unlike streamable-HTTP, the proxy cannot return a structured 502 for an individual bad event.

Proposed scope

A follow-up to #5288 scoped to the SSE response processor:

• Wrap the SSE body reader with a per-event interceptor
• On an invalid data: JSON-RPC frame: write a synthetic data: {"jsonrpc":"2.0","id":null,"error":{"code":-32000,"message":"Invalid upstream JSON-RPC frame"}}\n\n event and close the stream
• Leave non-data: SSE lines (comments, event:, retry:) untouched
• The guarantee is weaker than streamable-HTTP (client receives the error event, not a 502) — document this explicitly

Context

Found via DAST adversarial testing (checkMalformedFraming in the enterprise DAST suite). The test currently documents this as a known finding for the SSE transport.