fix(console): harden zen stream relay diagnostics

Add stream lifecycle metrics and extract the Zen relay loop so missing bodies, decoder stalls, EOF framing, and postprocess failures are easier to diagnose. Cover the new relay behavior with focused repro tests for the streaming edge cases we suspect in production.

packages/console/app/test/zenStream.test.ts

@@ -0,0 +1,139 @@
+import { describe, expect, test } from "bun:test"
+import { eventName, relay } from "../src/routes/zen/util/stream"
+
+const enc = new TextEncoder()
+
+const read = (stream: ReadableStream<Uint8Array>) => new Response(stream).text()
+
+const body = (parts: string[]) =>
+  new ReadableStream<Uint8Array>({
+    async start(c) {
+      for (const part of parts) c.enqueue(enc.encode(part))
+      c.close()
+    },
+  })
+
+describe("zen stream", () => {
+  test("parses known event names", () => {
+    expect(eventName("event: response.created\ndata: {}")).toBe("response.created")
+    expect(eventName('data: {"ok":true}')).toBe("message")
+    expect(eventName("data: [DONE]")).toBe("[DONE]")
+  })
+
+  test("relays split OpenAI responses and logs completion", async () => {
+    const seen: string[] = []
+    const logs: Array<Record<string, unknown>> = []
+    const stream = relay({
+      body: body([
+        "event: response.created\n",
+        'data: {"type":"response.created"}\n\n',
+        "event: response.completed\n",
+        'data: {"response":{"usage":{"input_tokens":1,"output_tokens":2}}}\n\n',
+      ]),
+      separator: "\n\n",
+      signal: new AbortController().signal,
+      start: Date.now(),
+      same: true,
+      parse: (part) => {
+        seen.push(part)
+      },
+      convert: (part) => part,
+      tail: async () => undefined,
+      metric: (values) => logs.push(values),
+    })
+
+    const text = await read(stream)
+    expect(text).toContain("response.created")
+    expect(text).toContain("response.completed")
+    expect(seen).toHaveLength(2)
+    expect(logs.at(-1)?.["stream.event"]).toBe("finished")
+    expect(logs.at(-1)?.["stream.saw_completed"]).toBe(true)
+  })
+
+  test("keeps reading when binary decoder needs another chunk", async () => {
+    let calls = 0
+    const logs: Array<Record<string, unknown>> = []
+    const stream = relay({
+      body: body(["a", "b"]),
+      separator: "\n\n",
+      signal: new AbortController().signal,
+      start: Date.now(),
+      same: true,
+      binary: (chunk) => {
+        calls += 1
+        if (calls === 1) return
+        return chunk
+      },
+      parse: () => undefined,
+      convert: (part) => part,
+      tail: async () => undefined,
+      metric: (values) => logs.push(values),
+    })
+
+    const text = await read(stream)
+    expect(text).toBe("b")
+    expect(logs.at(-1)?.["stream.event"]).toBe("finished")
+  })
+
+  test("flushes a final unterminated event at EOF", async () => {
+    const seen: string[] = []
+    const logs: Array<Record<string, unknown>> = []
+    const stream = relay({
+      body: body(['event: response.completed\ndata: {"response":{"usage":{"input_tokens":1}}}']),
+      separator: "\n\n",
+      signal: new AbortController().signal,
+      start: Date.now(),
+      same: true,
+      parse: (part) => {
+        seen.push(part)
+      },
+      convert: (part) => part,
+      tail: async () => undefined,
+      metric: (values) => logs.push(values),
+    })
+
+    const text = await read(stream)
+    expect(text).toContain("response.completed")
+    expect(seen).toHaveLength(1)
+    expect(logs.at(-1)?.["stream.saw_completed"]).toBe(true)
+  })
+
+  test("closes cleanly when upstream body is missing", async () => {
+    const logs: Array<Record<string, unknown>> = []
+    const stream = relay({
+      body: null,
+      separator: "\n\n",
+      signal: new AbortController().signal,
+      start: Date.now(),
+      same: true,
+      parse: () => undefined,
+      convert: (part) => part,
+      tail: async () => undefined,
+      metric: (values) => logs.push(values),
+    })
+
+    expect(await read(stream)).toBe("")
+    expect(logs.at(-1)?.["stream.event"]).toBe("missing_body")
+  })
+
+  test("surfaces postprocess failures with stream metrics", async () => {
+    const logs: Array<Record<string, unknown>> = []
+    const stream = relay({
+      body: body(["event: response.created\ndata: {}\n\n"]),
+      separator: "\n\n",
+      signal: new AbortController().signal,
+      start: Date.now(),
+      same: true,
+      parse: () => undefined,
+      convert: (part) => part,
+      tail: async () => {
+        throw new Error("boom")
+      },
+      metric: (values) => logs.push(values),
+    })
+
+    await expect(read(stream)).rejects.toThrow("boom")
+    expect(logs.at(-1)?.["stream.event"]).toBe("error")
+    expect(logs.at(-1)?.["stream.phase"]).toBe("tail")
+  })
+})