fix: prevent multistep invalid style leakage

[justin-schroeder]

Fixes #1208.

What changed

• Stops structural multi-step and step containers from exposing aggregate data-invalid/data-errors attributes.
• Keeps invalid state on the actual invalid child inputs, preventing parent multistep state from activating descendant invalid variants on valid fields.
• Adds a regression test that submits an invalid multistep form and verifies valid child inputs do not inherit parent invalid styling state.

Verification

• pnpm exec vitest run --config /tmp/formkit-vitest-ci.mts packages/addons/__tests__/multistep.spec.ts -t "#1208" --reporter verbose
• pnpm exec vitest run --config /tmp/formkit-vitest-ci.mts packages/addons/__tests__/multistep.spec.ts --reporter verbose
• pnpm exec vitest run --config /tmp/formkit-vitest-ci.mts packages/addons/__tests__ --reporter verbose
• pnpm vitest --typecheck.only --run

Local build note

• node scripts/cli.mjs --script build addons still fails during DTS generation on the existing local @formkit/auto-animate package exports/type-resolution issue.

Security

• No new user-controlled execution paths or HTML injection surfaces; this only removes aggregate state attributes from structural addon containers.