Fix malformed error message in ReadableStream.getReader

The error message for an invalid 'mode' argument in ReadableStream.getReader
was missing the "must be" part, resulting in a confusing error message like:
"The argument 'mode' byob. Received 'invalid'"

This fix updates the error message to properly say:
"The argument 'mode' must be 'byob'. Received 'invalid'"

A regression test has been added to ensure the error message is correct.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/js/builtins/ReadableStream.ts

@@ -395,7 +395,7 @@ export function getReader(this, options) {
     return new ReadableStreamBYOBReader(this);
   }
 
-  throw $ERR_INVALID_ARG_VALUE("mode", mode, "byob");
+  throw $ERR_INVALID_ARG_VALUE("mode", mode, "must be 'byob'");
 }
 
 export function pipeThrough(this, streams, options) {

test/js/web/streams/streams.test.js

@@ -1142,3 +1142,17 @@ it("pipeThrough doesn't cause unhandled rejections on readable errors", async ()
 
   expect(unhandledRejectionCaught).toBe(false);
 });
+
+it("ReadableStream.getReader with invalid mode should show correct error message", () => {
+  const stream = new ReadableStream();
+
+  // Test with invalid mode value
+  try {
+    stream.getReader({ mode: "invalid" });
+    expect.unreachable("Should have thrown an error");
+  } catch (error) {
+    expect(error.code).toBe("ERR_INVALID_ARG_VALUE");
+    expect(error.message).toContain("must be 'byob'");
+    expect(error.message).toContain("mode");
+  }
+});