test(tmux): add isolation regression coverage

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

bun-test.d.ts

@@ -1,14 +1,19 @@
 declare module "bun:test" {
+  type AnyFunction = (...args: any[]) => any
+
   interface MockMetadata<TArgs extends unknown[]> {
     calls: TArgs[]
   }
 
-  interface MockFunction<TArgs extends unknown[] = unknown[], TReturn = unknown> {
-    (...args: TArgs): TReturn
-    mock: MockMetadata<TArgs>
+  interface MockFunction<TFunction extends AnyFunction = AnyFunction> {
+    (...args: Parameters<TFunction>): ReturnType<TFunction>
+    mock: MockMetadata<Parameters<TFunction>>
+    mockClear(): void
     mockReset(): void
-    mockReturnValue(value: TReturn): void
-    mockResolvedValue(value: Awaited<TReturn>): void
+    mockRestore(): void
+    mockReturnValue(value: ReturnType<TFunction>): void
+    mockResolvedValue(value: Awaited<ReturnType<TFunction>>): void
+    mockImplementation(fn: TFunction): MockFunction<TFunction>
   }
 
   export function describe(name: string, fn: () => void): void
@@ -18,9 +23,12 @@ declare module "bun:test" {
   export function afterEach(fn: () => void | Promise<void>): void
   export function beforeAll(fn: () => void | Promise<void>): void
   export function afterAll(fn: () => void | Promise<void>): void
-  export function mock<TArgs extends unknown[], TReturn>(
-    fn: (...args: TArgs) => TReturn,
-  ): MockFunction<TArgs, TReturn>
+  export function mock<TFunction extends AnyFunction>(fn: TFunction): MockFunction<TFunction>
+
+  export function spyOn<TObject extends object>(
+    object: TObject,
+    key: keyof TObject,
+  ): MockFunction<AnyFunction>
 
   export namespace mock {
     function module(modulePath: string, factory: () => Record<string, unknown>): void