fix(session): accept legacy summary diffs (#26579)

Co-authored-by: Developer <temp@example.com>

packages/opencode/test/session/schema-decoding.test.ts

@@ -83,6 +83,26 @@ describe("Session.Info", () => {
     expect(Session.Info.zod.parse(input)).toEqual(input)
   })
 
+  test("accepts migrated summary diffs without file details", () => {
+    const input = {
+      id: sessionID,
+      slug: "legacy-diff",
+      projectID,
+      directory: "/tmp/proj",
+      title: "Legacy diff",
+      version: "0.1.0",
+      summary: {
+        additions: 1,
+        deletions: 0,
+        files: 1,
+        diffs: [{ additions: 1, deletions: 0 }],
+      },
+      time: { created: 1, updated: 2 },
+    }
+    expect(decode(input)).toEqual(input)
+    expect(Session.Info.zod.parse(input)).toEqual(input)
+  })
+
   test("rejects unbranded session id", () => {
     const bad = { id: "not-a-session-id" } as unknown
     expect(() => decode(bad)).toThrow()