Reset textarea scroll and caret after loading response schema example

[Copilot]

After clicking "load an example" in the Response constraint schema field, the textarea was populated but left scrolled to the end, obscuring the start of the JSON from the user.

built-in-ai/static/prompt-api.js

• After setting responseSchemaEl.value, reset focus, caret position, and scroll so the textarea opens at the top of the JSON:

responseSchemaEl.focus();
responseSchemaEl.setSelectionRange(0, 0);
responseSchemaEl.scrollTop = 0;
responseSchemaEl.scrollLeft = 0;