fix(sessions): preserve lifecycle headers across short reads (#109205)

src/config/sessions/lifecycle.ts

@@ -1,5 +1,6 @@
 // Session lifecycle timestamps prefer store metadata and fall back to transcript headers.
 import fs from "node:fs";
+import { readFileWindowFullySync } from "../../infra/file-read.js";
 import { asDateTimestampMs } from "../../shared/number-coercion.js";
 import { canonicalizeMainSessionAlias } from "./main-session.js";
 import {
@@ -111,7 +112,7 @@ function readFirstLine(filePath: string): string | undefined {
     const fd = fs.openSync(filePath, "r");
     try {
       const buffer = Buffer.alloc(8192);
-      const bytesRead = fs.readSync(fd, buffer, 0, buffer.length, 0);
+      const bytesRead = readFileWindowFullySync(fd, buffer, 0);
       if (bytesRead <= 0) {
         return undefined;
       }

src/config/sessions/sessions.test.ts

@@ -302,16 +302,34 @@ describe("session lifecycle timestamps", () => {
         "utf8",
       );
 
-      const timestamps = resolveSessionLifecycleTimestamps({
-        storePath,
-        entry: {
-          sessionId: "legacy-session",
-          sessionFile,
-          updatedAt: Date.parse("2026-04-25T08:00:00.000Z"),
-        },
-      });
+      const realReadSync = fs.readSync.bind(fs);
+      let shortReadCalls = 0;
+      const readSpy = vi.spyOn(fs, "readSync").mockImplementation(((
+        fd: number,
+        buffer: NodeJS.ArrayBufferView,
+        offset: number,
+        length: number,
+        position: fs.ReadPosition | null,
+      ) => {
+        shortReadCalls += 1;
+        return realReadSync(fd, buffer, offset, Math.min(length, 16), position);
+      }) as typeof fs.readSync);
 
-      expect(timestamps.sessionStartedAt).toBe(Date.parse(headerTimestamp));
+      try {
+        const timestamps = resolveSessionLifecycleTimestamps({
+          storePath,
+          entry: {
+            sessionId: "legacy-session",
+            sessionFile,
+            updatedAt: Date.parse("2026-04-25T08:00:00.000Z"),
+          },
+        });
+
+        expect(timestamps.sessionStartedAt).toBe(Date.parse(headerTimestamp));
+        expect(shortReadCalls).toBeGreaterThan(1);
+      } finally {
+        readSpy.mockRestore();
+      }
     } finally {
       await fsPromises.rm(dir, { recursive: true, force: true });
     }