feat: add SSE parser and in-flight request tracker

tests/sse.test.ts

@@ -0,0 +1,24 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { parseSse } from "../src/lib/sse";
+
+test("parseSse extracts event + data blocks separated by blank lines", () => {
+  const { messages, rest } = parseSse('event:message\ndata:{"a":1}\n\nevent:message\ndata:hello\n\n');
+  assert.equal(rest, "");
+  assert.equal(messages.length, 2);
+  assert.equal(messages[0].event, "message");
+  assert.equal(messages[0].data, '{"a":1}');
+  assert.equal(messages[1].data, "hello");
+});
+
+test("parseSse keeps a trailing partial event in rest", () => {
+  const { messages, rest } = parseSse("event:message\ndata:par");
+  assert.equal(messages.length, 0);
+  assert.equal(rest, "event:message\ndata:par");
+});
+
+test("parseSse joins multi-line data fields with newline", () => {
+  const { messages } = parseSse("data:line1\ndata:line2\n\n");
+  assert.equal(messages.length, 1);
+  assert.equal(messages[0].data, "line1\nline2");
+});