Review and enable SSE support for Rivet

[NathanFlurry]

Goal

Review and enable end-to-end Server-Sent Events (SSE) support for Rivet.

Implementation

• Review and merge feat(http): stream HTTP bodies end-to-end #5516
• Confirm HTTP request and response bodies stream end-to-end through Guard, the runner, Envoy, and Flume.
• Validate SSE behavior, including long-lived responses, cancellation/disconnect cleanup, backpressure, and observability.
• Add or confirm focused end-to-end coverage and update user-facing documentation where needed.

Dependency

Depends on #5484 / #5484 for Envoy protocol-version negotiation and compatibility.

Acceptance criteria

• PR #5516 is reviewed and merge-ready.
• SSE responses work end-to-end in Rivet.
• Envoy protocol compatibility is handled before rollout.
• Cancellation, disconnect, and backpressure behavior are covered.
• Relevant tests and documentation are complete.

[linear]

RVT-6386