fix(coding-agent): stream large session files

closes #5231

packages/coding-agent/test/session-manager/file-operations.test.ts

@@ -1,4 +1,5 @@
-import { mkdirSync, readFileSync, rmSync, writeFileSync } from "fs";
+import { constants as bufferConstants } from "buffer";
+import { appendFileSync, closeSync, mkdirSync, openSync, readFileSync, rmSync, writeFileSync, writeSync } from "fs";
 import { tmpdir } from "os";
 import { join } from "path";
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
@@ -63,6 +64,35 @@ describe("loadEntriesFromFile", () => {
 		const entries = loadEntriesFromFile(file);
 		expect(entries).toHaveLength(2);
 	});
+
+	it("opens session files larger than Node's max string length", () => {
+		const file = join(tempDir, "large.jsonl");
+		writeFileSync(
+			file,
+			'{"type":"session","version":3,"id":"abc","timestamp":"2025-01-01T00:00:00Z","cwd":"/tmp"}\n',
+		);
+
+		const fd = openSync(file, "r+");
+		try {
+			const newline = Buffer.from("\n");
+			const stride = 16 * 1024 * 1024;
+			for (let offset = stride; offset <= bufferConstants.MAX_STRING_LENGTH + stride; offset += stride) {
+				writeSync(fd, newline, 0, newline.length, offset);
+			}
+		} finally {
+			closeSync(fd);
+		}
+
+		appendFileSync(
+			file,
+			'{"type":"message","id":"1","parentId":null,"timestamp":"2025-01-01T00:00:01Z","message":{"role":"user","content":"hi","timestamp":1}}\n',
+		);
+
+		const sessionManager = SessionManager.open(file, tempDir);
+		expect(sessionManager.getSessionId()).toBe("abc");
+		expect(sessionManager.getEntries()).toHaveLength(1);
+		expect(sessionManager.buildSessionContext().messages).toEqual([{ role: "user", content: "hi", timestamp: 1 }]);
+	});
 });
 
 describe("findMostRecentSession", () => {