enable model selection via keyboard shortcut and refactor ModelSelect…

…Field

This commit introduces a keyboard shortcut (Cmd/Ctrl+M) to open the model selection dropdown in the webview.

The ModelSelectField component has been refactored to:

- Use a ref to directly control the popover's open/close state.
- Add an Escape key listener to close the dropdown.
- Remove the forwardRef and useImperativeHandle, simplifying the component's structure.
- Remove the openDropdownRef prop.
- Add a modelSelectorRef prop to allow parent components to control the dropdown.

These changes improve the user experience by providing a quick way to access the model selection and ensure the dropdown can be closed with the Escape key.

vscode/package.json

@@ -608,7 +608,7 @@
       },
       {
         "command": "cody.chat.new",
-        "key": "shift+alt+/",
+        "key": "cmd+n",
         "when": "cody.activated && !editorTextFocus"
       },
       {