fix: preserve submission callback rejections without a reason - #13967

Proposed Changes

handleSubmit(() => Promise.reject())() currently fulfills and sets isSubmitSuccessful to true. The callback failed, but its rejection reason is undefined, so the truthiness checks treat it as a successful submission.

Track whether onValid failed separately from its error value. Rethrow the original reason and keep isSubmitSuccessful false for synchronous throws and rejected promises with undefined, null, false, 0 or an empty string. Keep the existing submitted state and submit count behavior.

Added ten regression cases using the real form hook. This is separate from #13905's validation and onInvalid exception handling. No new dependencies or public API changes. No linked issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes

Validation on Node 22.23.4 and pnpm 11.28.10: 130 suites, 1,800 tests and 26 snapshots pass. Both type checks, builds, export checks, API Extractor and changed-file lint/format checks pass. Full lint has no errors and 572 existing warnings; API Extractor retains its TypeScript-version advisory. All 90 Chromium browser tests pass in the final full run. Reverting only the implementation makes all ten added cases fail; restoring it passes all 50 submit tests. The normal pre-commit checks pass.

Prepared with AI assistance (Codex and Instinct); the final two-file diff and validation evidence were reviewed. The local results apply to commit 5731b3a, based on 8eef3e0; current master 3f1257c merges cleanly, but the combined tree was not separately retested locally.