fix(cli): add missing custom theme properties to settings schema

- Add 'text.response', 'ui.active', and 'ui.focus' to CustomTheme schema.
- Synchronize 'CustomTheme' interface in core with schema by adding 'border.focused'.
- Add validation tests for the new properties.

Fixes https://github.com/google-gemini/gemini-cli/issues/25689

packages/core/src/config/config.ts

@@ -309,6 +309,7 @@ export interface CustomTheme {
   };
   border?: {
     default?: string;
+    focused?: string;
   };
   ui?: {
     comment?: string;