fix(form-core): handle null field defaults

[fenet-as]

Summary

• Fix isDefaultValue to respect an explicit field-level defaultValue: null.
• Update resetField to fall back to the form-level default only when the field-level default is undefined.
• Add a regression test covering field-level null defaults, value changes, and resets.

Fixes #2427

Summary by CodeRabbit

• Bug Fixes
  • Field-level defaultValue: null is now respected when checking whether a field has its default value and when resetting a field or form. Fields with undefined defaults continue to use the form-level default.

[changeset-bot]

🦋 Changeset detected

Latest commit: e7169e6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 14 packages

Name	Type
@tanstack/form-core	Patch
@tanstack/angular-form	Patch
@tanstack/form-devtools	Patch
@tanstack/lit-form	Patch
@tanstack/preact-form	Patch
@tanstack/react-form	Patch
@tanstack/solid-form	Patch
@tanstack/svelte-form	Patch
@tanstack/vue-form	Patch
@tanstack/react-form-devtools	Patch
@tanstack/solid-form-devtools	Patch
@tanstack/react-form-nextjs	Patch
@tanstack/react-form-remix	Patch
@tanstack/react-form-start	Patch

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository: TanStack/form/.coderabbit.yaml
• Review profile: CHILL
• Plan: Advanced
• Run ID: 07ff809e-c847-46a6-9b72-3cf2bb2cc6e0

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📥 Commits

Reviewing files that changed from the base of the PR and between 2216fde and e7169e6.

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📒 Files selected for processing (3)

• .changeset/null-field-default-value.md
• packages/form-core/src/FormApi.ts
• packages/form-core/tests/FieldApi.spec.ts

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📝 Walkthrough

Walkthrough

Field-level defaultValue: null is now used when resolving isDefaultValue and when resetField selects a value. Tests cover value changes, field reset, and form reset. A patch changeset describes the behavior.

Changes

Field Default Resolution

Layer / File(s)	Summary
Resolve and reset nullable field defaults packages/form-core/src/FormApi.ts, packages/form-core/tests/FieldApi.spec.ts, .changeset/null-field-default-value.md	Default-status checks and resetField use a field-level null default. The tests verify the default status after value changes, field reset, and form reset. The changeset records the update.

Priority: ⬇️ Low

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

Change: Bug fix · Severity of issue fixed: Low

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Merge Risk: ⚪ Minimal · up to e7169

No actionable issue remains from this review; the change appears ready to merge after normal checks.

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check		The description explains the change, motivation, regression test, and linked issue. It does not use the required Changes, Checklist, or Release Impact sections, and it omits the required checklist and…	Rewrite the description using the repository template. Add the Changes, Checklist, and Release Impact sections, then mark the applicable checklist items and confirm the changeset status.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: handling null field defaults in form-core.
Linked Issues check		The PR addresses the coding requirements in #2427. FormApi now treats a field-level defaultValue: null as the selected default and uses the form-level default only when the field default is `undef…
Out of Scope Changes check		The changes stay within the linked issue scope. The FormApi fix implements null field-default handling, the FieldApi test verifies the regression, and the form-core changeset documents the behavio…
Docstring Coverage		No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…

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Full details: Description check

Explanation

The description explains the change, motivation, regression test, and linked issue. It does not use the required Changes, Checklist, or Release Impact sections, and it omits the required checklist and release-impact selections.

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