Add SSE response summary helpers (#466)

Co-authored-by: Gregory Schier <gschier1990@gmail.com>

crates/yaak-sse/summary.test.ts

@@ -0,0 +1,51 @@
+import { describe, expect, it } from "vitest";
+import { computeSseSummary, extractSseValueAtPath } from "./summary";
+
+describe("extractSseValueAtPath", () => {
+  it("supports simple paths", () => {
+    expect(
+      extractSseValueAtPath(
+        JSON.stringify({ choices: [{ delta: { content: "hello" } }] }),
+        "$.choices[0].delta.content",
+      ),
+    ).toBe("hello");
+  });
+
+  it("supports full JSONPath expressions", () => {
+    expect(
+      extractSseValueAtPath(
+        JSON.stringify({
+          choices: [
+            { delta: { role: "assistant" } },
+            { delta: { content: "hello" } },
+            { delta: { content: " world" } },
+          ],
+        }),
+        "$.choices[*].delta.content",
+      ),
+    ).toBe("hello world");
+  });
+
+  it("returns null when a JSONPath expression has no matches", () => {
+    expect(extractSseValueAtPath(JSON.stringify({ delta: {} }), "$.delta.text")).toBeNull();
+  });
+});
+
+describe("computeSseSummary", () => {
+  it("concatenates JSONPath matches across SSE messages", () => {
+    expect(
+      computeSseSummary(
+        [
+          `data: ${JSON.stringify({ choices: [{ delta: { content: "hello" } }] })}`,
+          "",
+          `data: ${JSON.stringify({ choices: [{ delta: { content: " world" } }] })}`,
+          "",
+        ].join("\n"),
+        "$.choices[*].delta.content",
+      ),
+    ).toEqual({
+      fragmentCount: 2,
+      summary: "hello world",
+    });
+  });
+});