fix(cursor): honor legacy read range and grep offset

Modern Cursor builds paginate the legacy `read` and `grep` frames with
fields this branch modeled in the proto but never wired.

`read` composed no range, so every page returned the whole file (or its
own truncation) and a model walking a large file never advanced past the
first window. It now goes through `piReadPath`, the same helper the Pi
frame uses, so both translate a range identically - including the
`limit: 0` case, which asks for zero lines and has no selector. The
answer reports `range_applied`, left false for an unranged read since
that is precisely the server's "this is the whole file".

`grep` dropped its `offset`. The local tool paginates by file through
`skip` and advertises exactly that unit in its own "use skip=N" advice,
so an unforwarded offset re-ran the identical search and answered page
one forever. A present `0` stays unset: it means "start at the
beginning", which is the un-skipped search.

(cherry picked from commit 4f647d86b6e4a507d57fb4d247f19db8240e51cf)

packages/ai/test/cursor-exec-modern.test.ts

@@ -1533,3 +1533,41 @@ describe("Cursor modern exec frames: server-resolved tool calls leave a paired b
 		expect(output.content.filter(block => block.type === "toolCall")).toHaveLength(0);
 	});
 });
+
+describe("Cursor legacy read frame: range reporting", () => {
+	it("reports rangeApplied only when the frame asked for a window", async () => {
+		// `range_applied: false` tells the server "this is the whole file", which
+		// is true for an unranged read and a lie for a paginated one — a model
+		// walking a large file would stop after the first window believing it had
+		// seen everything.
+		const handlers: CursorExecHandlers = {
+			async read() {
+				return toolResult("line1\nline2");
+			},
+		};
+
+		const ranged = await dispatchExec(
+			buildExecMessage({
+				case: "readArgs",
+				value: create(ReadArgsSchema, { path: "/repo/big.ts", toolCallId: "c1", offset: 5, limit: 20 }),
+			}),
+			{ execHandlers: handlers },
+		);
+		const rangedAnswer = soleResult(ranged.frames);
+		if (rangedAnswer.case !== "readResult") throw new Error(`got ${rangedAnswer.case}`);
+		if (rangedAnswer.value.result.case !== "success") throw new Error(`got ${rangedAnswer.value.result.case}`);
+		expect(rangedAnswer.value.result.value.rangeApplied).toBe(true);
+
+		const whole = await dispatchExec(
+			buildExecMessage({
+				case: "readArgs",
+				value: create(ReadArgsSchema, { path: "/repo/big.ts", toolCallId: "c2" }),
+			}),
+			{ execHandlers: handlers },
+		);
+		const wholeAnswer = soleResult(whole.frames);
+		if (wholeAnswer.case !== "readResult") throw new Error(`got ${wholeAnswer.case}`);
+		if (wholeAnswer.value.result.case !== "success") throw new Error(`got ${wholeAnswer.value.result.case}`);
+		expect(wholeAnswer.value.result.value.rangeApplied).toBe(false);
+	});
+});