fix: apply defaults to untouched selects with disabled optgroups - #13953

Proposed Changes

An untouched single select with its first option inside a disabled optgroup is mistaken for a pre-hydration edit. The browser skips that option, but its own disabled property remains false. RHF then retains the browser-selected value instead of applying defaultValues and marks the field dirty. For example, a disabled group containing A followed by B/C retains B despite defaultValues: { choice: 'c' }.

Exclude options whose parent is a disabled optgroup when finding the browser's initial enabled option. Real user edits remain preserved. This is a follow-up to #13872, found while reviewing that path; no issue was filed. It does not change reset's disabled-field registry or add a public API/dependency.

Type of change

• Bug fix

Checklist

• Code follows the project style
• Self-reviewed the focused diff
• No new warnings in changed files
• Added tests proving the fix and preserving user edits
• New and existing tests pass locally

Validation on Node 22.23.2 and pnpm 11.21.0, based on master a8e1103:

• Source-only revert: one regression fails, four added controls pass. Patched: 129 suites, 1,767 tests, 26 snapshots pass.
• TypeScript, type tests, build/export checks, API extraction, formatting and lint pass. Full lint reports 570 warnings; changed files have none. API Extractor retains its TS compiler-version advisory.
• All 90 existing browser tests pass using installed Brave. A separate 12-case scratch browser matrix passes hydration, dirty-state and actual submit-payload checks for disabled/enabled optgroups, directly disabled options, multiple selects, listboxes and explicit server selections, with/without pre-hydration edits.
• Initial scratch browser run had two invalid-hook failures during lazy React server-renderer optimization. Explicit initial Vite preoptimization resolved the harness issue; all 12 cases then passed. No library changes were made for that harness correction. Browser tests used a dedicated local port and the freshly built package; other browsers were not tested.

AI-assisted implementation and verification with Codex; Jev supplied bounded engineering decisions.