fix(test): use -S flag in shebang for Linux compatibility

The shebang `#!/usr/bin/env bun run` works on macOS but fails on Linux
because the `env` command doesn't split space-separated arguments by
default. Using `#!/usr/bin/env -S bun run` makes env parse the string
as multiple arguments, enabling cross-platform compatibility.

Fixes CI failures where the mock server couldn't start due to:
  /usr/bin/env: 'bun run': No such file or directory

Author: ryoppippi

tests/mocks/serve.ts

@@ -1,4 +1,4 @@
-#!/usr/bin/env bun run
+#!/usr/bin/env -S bun run
 /**
  * Standalone HTTP server for MCP mock testing.
  * Imports createMcpApp from stackone-ai-node vendor submodule.