SSE streaming for real-time agent execution updates

[fractlrao]

Summary

• Adds Server-Sent Events (SSE) streaming so API clients receive real-time progress updates during agent execution
• Clients opt in by sending Accept: text/event-stream on POST event endpoints — non-streaming requests are completely unchanged
• Event types are plain strings (not a fixed enum), so any part of the runtime can emit custom events on the fly
• Includes unit tests for the SSE emitter and environment integration, plus an end-to-end integration test

Use cases

Chat UIs and dashboards — Instead of showing a blank spinner for 10-30 seconds while an agent runs, a frontend can display live progress: which flow step is executing, when the LLM responds, and intermediate results as they arrive.

Multi-step agent flows — When an agent orchestrates a flow with multiple steps (data retrieval, decision-making, API calls), each step transition streams as a flow_step_start/flow_step_complete event. A client can render a live step-by-step timeline.

Debugging and observability — Developers building Agentlang apps can curl -N with the SSE header and watch the full execution trace in real time, seeing exactly where time is spent and where errors occur.

Error handling mid-stream — If an LLM call fails partway through a multi-step flow, the client receives the error event immediately rather than waiting for the full timeout, enabling faster user feedback and retry logic.

Long-running agent tasks — For agents that perform complex planning with retries, the client stays informed throughout instead of facing an opaque wait.

How it works

• New SseEmitter class (src/api/sse.ts) wraps an Express Response for SSE protocol: headers, keep-alive, event formatting, and cleanup
• Environment gains setSseEmitter() / emitSseEvent() — the emitter propagates from parent to child environments automatically
• emitSseEvent is null-safe: no-ops when no emitter is set, so zero impact on non-streaming paths
• Events emitted at key points: agent start/complete, flow step transitions, LLM responses (with token usage), and errors
• handleEventPost in http.ts forks on the Accept header — SSE path creates an emitter environment, non-SSE path is untouched

Test plan

• SseEmitter unit tests: headers, event format, arbitrary event types, close idempotency, client disconnect
• isStreamingRequest tests: various Accept header values
• Environment.emitSseEvent tests: no-op without emitter, emit with emitter, no-op when closed, child inheritance
• Integration test: evaluate() with SSE emitter through full runtime bootstrap
• All 120 existing tests pass with no regressions
• Manual test: curl -N -H "Accept: text/event-stream" against a running app with an agent event