fix(form-core): keep onSubmit error when blurring an unchanged field

The submit-error auto-clear in FieldApi.validateSync fired on every
non-submit validation cause, including blur. As a result, focusing and
leaving a field without editing it dropped a valid submit error.

Gate the clear on a value `change` cause so the error is only removed
when the user actually enters a new value, matching the documented
intent of the block.

Closes #1242

Author: chatman-media

packages/form-core/src/FieldApi.ts

@@ -1347,14 +1347,17 @@ export class FieldApi<
 
     /**
      *  when we have an error for onSubmit in the state, we want
-     *  to clear the error as soon as the user enters a valid value in the field
+     *  to clear the error as soon as the user enters a valid value in the field.
+     *  This must only happen on a value `change` - clearing it on `blur` (or any
+     *  other non-value cause) would wrongly drop the submit error when the user
+     *  focuses and leaves the field without editing it.
      */
     const submitErrKey = getErrorMapKey('submit')
 
     if (
       // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
       this.state.meta.errorMap?.[submitErrKey] &&
-      cause !== 'submit' &&
+      cause === 'change' &&
       !hasErrored
     ) {
       this.setMeta((prev) => ({

packages/form-core/tests/FieldApi.spec.ts

@@ -2098,6 +2098,64 @@ describe('field api', () => {
     expect(field.getMeta().errors).toStrictEqual(['first name is required'])
   })
 
+  it('should not clear onSubmit errors on blur when the value did not change', async () => {
+    const form = new FormApi({
+      defaultValues: {
+        firstName: '',
+      },
+    })
+
+    form.mount()
+
+    const field = new FieldApi({
+      form,
+      name: 'firstName',
+      validators: {
+        onSubmit: ({ value }) =>
+          value.length > 0 ? undefined : 'first name is required',
+      },
+    })
+
+    field.mount()
+
+    await form.handleSubmit()
+    expect(field.getMeta().errors).toStrictEqual(['first name is required'])
+
+    // Blurring the field without changing its value must keep the submit error
+    field.handleBlur()
+    expect(field.getMeta().errors).toStrictEqual(['first name is required'])
+    expect(field.getMeta().isValid).toBe(false)
+  })
+
+  it('should clear onSubmit errors once a valid value is entered', async () => {
+    const form = new FormApi({
+      defaultValues: {
+        firstName: '',
+      },
+    })
+
+    form.mount()
+
+    const field = new FieldApi({
+      form,
+      name: 'firstName',
+      validators: {
+        onSubmit: ({ value }) =>
+          value.length > 0 ? undefined : 'first name is required',
+      },
+    })
+
+    field.mount()
+
+    await form.handleSubmit()
+    expect(field.getMeta().errors).toStrictEqual(['first name is required'])
+
+    // Entering a valid value clears the stale submit error
+    field.handleChange('John')
+    expect(field.getMeta().errors).toStrictEqual([])
+    expect(field.getMeta().isValid).toBe(true)
+  })
+
   it('should show onMount errors', async () => {
     const form = new FormApi({
       defaultValues: {