fix(form-core): apply async defaultValues to untouched fields after other fields are edited

[xianjianlf2]

🎯 Changes

Fixes #2229. FormApi.update() previously gated default-value merging on the form-level touched state, so touching one field prevented asynchronously arriving defaults from applying to every other untouched field.

• Drop the form-level touched gate and merge updated defaults per field instead.
• Preserve user-edited values for touched fields.
• Apply new default values to untouched fields, including fields that have not mounted yet.
• Add regression coverage for both scalar fields and array fields.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.

Notes

The existing behavior of not wiping an edited field during update is preserved. The new behavior only applies newly arrived defaults to fields that are still untouched.

[coderabbitai]

📝 Walkthrough

Walkthrough

FormApi.update now merges incoming default values with current values, preserving touched fields while updating untouched fields. Tests cover scalar and array defaults, and a patch changeset documents the fix.

Changes

Async default value updates

Layer / File(s)	Summary
Preserve edits during default updates packages/form-core/src/FormApi.ts, .changeset/async-default-untouched-fields.md	FormApi.update applies changed defaults to untouched fields while retaining current values for touched fields.
Validate scalar and array defaults packages/form-core/tests/FormApi.spec.ts	Tests verify that scalar and array defaults update correctly without overwriting edited values.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• TanStack/form#2190: Both changes address async defaultValues updates, including array-field update behavior.

Suggested reviewers: crutchcorn

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The code and tests satisfy #2229 by merging async defaults per field and preserving edited values.
Out of Scope Changes check	✅ Passed	The changes stay within the issue scope: implementation fix, regression tests, and a changeset entry.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title clearly matches the main change: async defaultValues now apply to untouched fields after edits.
Description check	✅ Passed	The description follows the template and includes changes, checklist, release impact, and a changeset note.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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