fix(cli): allow null path in screenshot command validation (#101)

The Rust CLI sends `null` for the `path` argument when it is not provided,
but the Zod schema only accepted `undefined`. This change updates the
`screenshotSchema` to allow `null` values for `path`, enabling the
`screenshot` command to work without a file path argument (outputting to stdout).

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

src/protocol.ts

@@ -670,7 +670,7 @@ const pressSchema = baseCommandSchema.extend({
 
 const screenshotSchema = baseCommandSchema.extend({
   action: z.literal('screenshot'),
-  path: z.string().optional(),
+  path: z.string().nullable().optional(),
   fullPage: z.boolean().optional(),
   selector: z.string().min(1).optional(),
   format: z.enum(['png', 'jpeg']).optional(),