fix: fail loud when PTY cursor mode is unknown (#51490) (thanks @liuy)

* fix(process): auto-detect PTY cursor key mode for send-keys

When a PTY session sends smkx (\x1b[?1h) or rmkx (\x1b[?1l) to switch
cursor key mode, send-keys now detects this and encodes cursor keys
accordingly.

- smkx/rmkx detection in handleStdout before sanitizeBinaryOutput
- cursorKeyMode stored in ProcessSession
- encodeKeySequence accepts cursorKeyMode parameter
- DECCKM_SS3_KEYS for application mode (arrows + home/end)
- CSI sequences for normal mode
- Modified keys (including alt) always use xterm modifier scheme
- Extract detectCursorKeyMode for unit testing
- Use lastIndexOf to find last toggle in chunk (later one wins)

Fixes #51488

* fix: fail loud when PTY cursor mode is unknown (#51490) (thanks @liuy)

* style: format process send-keys guard (#51490) (thanks @liuy)

---------

Co-authored-by: Ayaan Zaidi <hi@obviy.us>

src/agents/bash-tools.exec-runtime.test.ts

@@ -4,9 +4,42 @@ const requestHeartbeatNowMock = vi.hoisted(() => vi.fn());
 const enqueueSystemEventMock = vi.hoisted(() => vi.fn());
 
 let buildExecExitOutcome: typeof import("./bash-tools.exec-runtime.js").buildExecExitOutcome;
+let detectCursorKeyMode: typeof import("./bash-tools.exec-runtime.js").detectCursorKeyMode;
 let emitExecSystemEvent: typeof import("./bash-tools.exec-runtime.js").emitExecSystemEvent;
 let formatExecFailureReason: typeof import("./bash-tools.exec-runtime.js").formatExecFailureReason;
 
+describe("detectCursorKeyMode", () => {
+  beforeEach(async () => {
+    ({ detectCursorKeyMode } = await import("./bash-tools.exec-runtime.js"));
+  });
+
+  it("returns null when no toggle found", () => {
+    expect(detectCursorKeyMode("hello world")).toBe(null);
+    expect(detectCursorKeyMode("")).toBe(null);
+  });
+
+  it("detects smkx (application mode)", () => {
+    expect(detectCursorKeyMode("\x1b[?1h")).toBe("application");
+    expect(detectCursorKeyMode("\x1b[?1h\x1b=")).toBe("application");
+    expect(detectCursorKeyMode("before \x1b[?1h after")).toBe("application");
+  });
+
+  it("detects rmkx (normal mode)", () => {
+    expect(detectCursorKeyMode("\x1b[?1l")).toBe("normal");
+    expect(detectCursorKeyMode("\x1b[?1l\x1b>")).toBe("normal");
+    expect(detectCursorKeyMode("before \x1b[?1l after")).toBe("normal");
+  });
+
+  it("last toggle wins when both present", () => {
+    // smkx first, then rmkx - should be normal
+    expect(detectCursorKeyMode("\x1b[?1h\x1b[?1l")).toBe("normal");
+    // rmkx first, then smkx - should be application
+    expect(detectCursorKeyMode("\x1b[?1l\x1b[?1h")).toBe("application");
+    // Multiple toggles - last one wins
+    expect(detectCursorKeyMode("\x1b[?1h\x1b[?1l\x1b[?1h")).toBe("application");
+  });
+});
+
 describe("emitExecSystemEvent", () => {
   beforeEach(async () => {
     vi.resetModules();