test(extensions): add failing tests to expose broken autofix behavior

Follow-up to #391

These tests demonstrate that the `import-x/extensions` rule only applies autofixes when
both `fix: true` and a `pattern` object are set—even if the pattern is empty. This behavior
is unintuitive, undocumented, and inconsistent with how autofix typically works in ESLint.

The tests assert on the `output` field to document the current (broken) behavior and provide
a basis for resolving the issue.

In addition to fixing the logic, I recommend removing the `fix` option entirely, as it is
undocumented and not used in any other rule in the plugin.

Author: lnhrdt

test/rules/extensions.spec.ts

@@ -173,6 +173,73 @@ ruleTester.run('extensions', rule, {
   ],
 
   invalid: [
+    tInvalid({
+      name: 'extensions should autofix by default',
+      code: 'import a from "a/foo.js"',
+      options: ['never'],
+      errors: [
+        {
+          messageId: 'unexpected',
+          data: { extension: 'js', importPath: 'a/foo.js' },
+          line: 1,
+          column: 15,
+        },
+      ],
+      output: 'import a from "a/foo"',
+    }),
+    tInvalid({
+      name: 'extensions should autofix when fix is set to true',
+      code: 'import a from "a/foo.js"',
+      options: ['never', {fix: true}],
+      errors: [
+        {
+          messageId: 'unexpected',
+          data: { extension: 'js', importPath: 'a/foo.js' },
+          line: 1,
+          column: 15,
+        },
+      ],
+      output: 'import a from "a/foo"',
+    }),
+    tInvalid({
+      name: 'extensions should autofix when fix is set to true and a pattern object is provided',
+      code: 'import a from "a/foo.js"',
+      options: ['never', {fix: true, pattern: {}}],
+      errors: [
+        {
+          messageId: 'unexpected',
+          data: { extension: 'js', importPath: 'a/foo.js' },
+          line: 1,
+          column: 15,
+        },
+      ],
+      output: 'import a from "a/foo"',
+    }),
+    tInvalid({
+      name: 'extensions should not autofix when fix is set to false',
+      code: 'import a from "a/foo.js"',
+      options: ['never', {fix: false}],
+      errors: [
+        {
+          messageId: 'unexpected',
+          data: { extension: 'js', importPath: 'a/foo.js' },
+          line: 1,
+          column: 15,
+          suggestions: [
+            {
+              messageId: 'removeUnexpected',
+              data: {
+                extension: 'js',
+                importPath: 'a/foo.js',
+                fixedImportPath: 'a/foo',
+              },
+              output: 'import a from "a/foo"',
+            },
+          ],
+        },
+      ],
+      output: null,
+    }),
     tInvalid({
       code: 'import a from "a/index.js"',
       errors: [