Fix GlFormFields rendering issue

What does this MR do?

Related to #2418

There are two rendering issues with GlFormFields:

1. When the component is re-rendered the id attribute of inputs is incremented. e.g gl-form-field-1 becomes gl-form-field-2.
2. There is no key on the v-for so when the fields are re-rendered they can lose focus. See #2418

This MR does a few things:

1. Uses field name for id attribute instead of uniqueId to avoid incrementing every time component is re-rendered
2. Adds support for passing a custom id to a field input
3. Adds a key to v-for to fix focus/rendering issue when adding fields dynamically

Screenshots or screen recordings

Before	After
Screen_Recording_2025-10-16_at_1.46.50_PM	Screen_Recording_2025-10-23_at_11.08.24_AM

How to test locally

1. Apply this patch

diff --git a/packages/gitlab-ui/src/components/base/form/form_fields/form_fields.stories.js b/packages/gitlab-ui/src/components/base/form/form_fields/form_fields.stories.js
index f323f037d..86f1c1fc9 100644
--- a/packages/gitlab-ui/src/components/base/form/form_fields/form_fields.stories.js
+++ b/packages/gitlab-ui/src/components/base/form/form_fields/form_fields.stories.js
@@ -89,6 +89,9 @@ const Template = () => ({
       return null;
     },
   },
+  created() {
+    setInterval(this.addField, 5000);
+  },
   methods: {
     onInputField({ name }) {
       this.$delete(this.serverValidations, name);
@@ -113,6 +116,17 @@ const Template = () => ({
 
       this.$refs.modal.show();
     },
+    addField() {
+      const fieldId = uniqueId();
+
+      this.fields = {
+        [`field${fieldId}`]: {
+          label: `Extra Field ${fieldId}`,
+          id: `custom-form-field-id-${fieldId}`,
+        },
+        ...this.fields,
+      };
+    },
   },
   template: `
     <div>

2. Run yarn storybook
3. Go to http://localhost:9001/iframe.html?args=&id=base-form-form-fields--default&viewMode=story

Integrations

• GitLab: gitlab-org/gitlab!209530
• CustomersDot: GlFormFields not used
• Duo UI: GlFormFields not used
• Status Page: GlFormFields not used
• Docs: GlFormFields not used
• Switchboard: GlFormFields not used

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