fix(ui): reconnect closed SSE streams (#362)

## Summary
- Reconnect the UI event stream when a runtime surfaces an SSE close
notification, not only on EventSource errors.
- Avoid scheduling duplicate reconnect loops when close/error
notifications arrive together.
- Add a targeted EventSource handler test for the close paths described
in #207.

## Validation
- node --experimental-strip-types --test
"packages/ui/src/lib/event-source-handlers.test.ts"
- npm run build --workspace @codenomad/ui

Closes #207

packages/ui/src/lib/event-source-handlers.test.ts

@@ -0,0 +1,69 @@
+import assert from "node:assert/strict"
+import { describe, it } from "node:test"
+import { attachEventSourceHandlers } from "./event-source-handlers.ts"
+
+class FakeEventSource extends EventTarget {
+  onmessage: ((event: MessageEvent) => void) | null = null
+  onerror: (() => void) | null = null
+  onclose: (() => void) | null = null
+}
+
+const logger = {
+  warn() {},
+  error() {},
+}
+
+describe("attachEventSourceHandlers", () => {
+  it("requests reconnect when EventSource emits close", () => {
+    const source = new FakeEventSource()
+    let reconnects = 0
+
+    attachEventSourceHandlers(source as unknown as EventSource, {
+      onEvent() {},
+      onError: () => {
+        reconnects += 1
+      },
+      logger,
+    })
+
+    source.dispatchEvent(new Event("close"))
+
+    assert.equal(reconnects, 1)
+  })
+
+  it("requests reconnect when EventSource invokes onclose", () => {
+    const source = new FakeEventSource()
+    let reconnects = 0
+
+    attachEventSourceHandlers(source as unknown as EventSource, {
+      onEvent() {},
+      onError: () => {
+        reconnects += 1
+      },
+      logger,
+    })
+
+    source.onclose?.()
+
+    assert.equal(reconnects, 1)
+  })
+
+  it("requests reconnect once when a close notification hits multiple handlers", () => {
+    const source = new FakeEventSource()
+    let reconnects = 0
+
+    attachEventSourceHandlers(source as unknown as EventSource, {
+      onEvent() {},
+      onError: () => {
+        reconnects += 1
+      },
+      logger,
+    })
+
+    source.onclose?.()
+    source.dispatchEvent(new Event("close"))
+    source.onerror?.()
+
+    assert.equal(reconnects, 1)
+  })
+})