fix(test): use vi.fn() for TTY method stubs instead of plain functions

Changed stub creation from plain functions `() => {}` to `vi.fn()` to prevent
double-counting of mock calls. When spying on a plain function stub, some test
environments were counting both the spy call and the underlying stub call,
causing tests to fail with "expected 2 calls, got 5" errors.

Using vi.fn() creates a proper mock function that integrates correctly with
vitest's spy system, ensuring spy call counts are accurate across all platforms.

Fixes test failures in test/stdio/stderr.test.ts and test/stdio/stdout.test.ts
that occurred in CI environments.

Author: jdalton

test/stdio/stderr.test.ts

@@ -32,19 +32,20 @@ describe('stdio/stderr', () => {
     originalColumns = stderr.columns
     originalRows = stderr.rows
 
-    // Add TTY methods if they don't exist (for non-TTY environments)
-    if (!stderr.cursorTo) {
-      ;(stderr as any).cursorTo = () => {}
-    }
-    if (!stderr.clearLine) {
-      ;(stderr as any).clearLine = () => {}
-    }
-
-    // Create spies
+    // Create spies (add methods if they don't exist in non-TTY environments)
     // @ts-expect-error - Vitest spy type doesn't match ReturnType<typeof vi.spyOn>
     writeSpy = vi.spyOn(stderr, 'write').mockImplementation(() => true)
+
+    // Create stubs for TTY methods only if they don't exist, then spy on them
+    if (!stderr.cursorTo) {
+      ;(stderr as any).cursorTo = vi.fn()
+    }
     // @ts-expect-error - Vitest spy type doesn't match ReturnType<typeof vi.spyOn>
     cursorToSpy = vi.spyOn(stderr, 'cursorTo').mockImplementation(() => {})
+
+    if (!stderr.clearLine) {
+      ;(stderr as any).clearLine = vi.fn()
+    }
     // @ts-expect-error - Vitest spy type doesn't match ReturnType<typeof vi.spyOn>
     clearLineSpy = vi.spyOn(stderr, 'clearLine').mockImplementation(() => {})
   })

test/stdio/stdout.test.ts

@@ -36,27 +36,29 @@ describe('stdio/stdout', () => {
     originalColumns = stdout.columns
     originalRows = stdout.rows
 
-    // Add TTY methods if they don't exist (for non-TTY environments)
-    if (!stdout.cursorTo) {
-      ;(stdout as any).cursorTo = () => {}
-    }
-    if (!stdout.clearLine) {
-      ;(stdout as any).clearLine = () => {}
-    }
-    if (!stdout.clearScreenDown) {
-      ;(stdout as any).clearScreenDown = () => {}
-    }
-
     // Make stdout appear as a WriteStream instance for hide/showCursor tests
     Object.setPrototypeOf(stdout, WriteStream.prototype)
 
-    // Create spies
+    // Create spies (add methods if they don't exist in non-TTY environments)
     // @ts-expect-error - Vitest spy type doesn't match ReturnType<typeof vi.spyOn>
     writeSpy = vi.spyOn(stdout, 'write').mockImplementation(() => true)
+
+    // Create stubs for TTY methods only if they don't exist, then spy on them
+    if (!stdout.cursorTo) {
+      ;(stdout as any).cursorTo = vi.fn()
+    }
     // @ts-expect-error - Vitest spy type doesn't match ReturnType<typeof vi.spyOn>
     cursorToSpy = vi.spyOn(stdout, 'cursorTo').mockImplementation(() => {})
+
+    if (!stdout.clearLine) {
+      ;(stdout as any).clearLine = vi.fn()
+    }
     // @ts-expect-error - Vitest spy type doesn't match ReturnType<typeof vi.spyOn>
     clearLineSpy = vi.spyOn(stdout, 'clearLine').mockImplementation(() => {})
+
+    if (!stdout.clearScreenDown) {
+      ;(stdout as any).clearScreenDown = vi.fn()
+    }
     // @ts-expect-error - Vitest spy type doesn't match ReturnType<typeof vi.spyOn>
     clearScreenDownSpy = vi
       .spyOn(stdout, 'clearScreenDown')