test: validate non-string file paths in parseFileTemplateVariables

Add test cases to verify that `parseFileTemplateVariables` correctly
throws an error when a non-string value (e.g. number, boolean, object)
is provided as a file path in the input YAML. This ensures the existing
validation is properly tested.

Co-authored-by: Pet3cy <169947521+Pet3cy@users.noreply.github.com>

__tests__/prompt.test.ts

@@ -135,5 +135,17 @@ describe('prompt.ts', () => {
     it('errors on missing files', () => {
       expect(() => parseFileTemplateVariables('x: ./does-not-exist.txt')).toThrow('was not found')
     })
+
+    it('errors on non-string file paths', () => {
+      expect(() => parseFileTemplateVariables('x: 123')).toThrow(
+        "File template variable 'x' must be a string file path",
+      )
+      expect(() => parseFileTemplateVariables('x: true')).toThrow(
+        "File template variable 'x' must be a string file path",
+      )
+      expect(() => parseFileTemplateVariables('x: { nested: "object" }')).toThrow(
+        "File template variable 'x' must be a string file path",
+      )
+    })
   })
 })