"Skip and finish" cancels the whole setup instead of skipping the step and finishing

Problem/Motivation

When the full-setup flow produces more than one step — a validation plugin plus at least one @TfaLogin plugin (e.g. tfa_trusted_browser) — the "Skip" / "Skip and finish" button cancels the entire setup instead of skipping the current step and finishing. In TfaSetupForm::buildForm() the skip button carries '#submit' => ['::cancelForm']:

$form['actions']['skip'] = [
  '#type' => 'submit',
  '#value' => $count > 0 ? $this->t('Skip') : $this->t('Skip and finish'),
  '#limit_validation_errors' => [],
  '#submit' => ['::cancelForm'],
];

So pressing it runs cancelForm() ("Setup of ... canceled.") and aborts the whole setup. The dedicated skip branch already present in submitForm() (isset($values['skip']) && $values['op'] === $values['skip']) is therefore dead code and never runs.

Steps to reproduce

Reproduced on 8.x-1.12 (the same code is present on 8.x-1.x) with:

• default_validation_plugin: tfa_totp
• login_plugins: { tfa_trusted_browser }

which yields the two-step sequence [tfa_totp, tfa_trusted_browser].

Precondition: the multi-step path is only reachable once the setup-plugin re-resolve bug (#3451488, MR !163) is fixed — on a pristine build the user never advances past the first (TOTP) step, so this bug is not observable. Apply MR !163 first, then use the configuration above.

1. As a fresh user with no TFA configured, open /user/{uid}/security/tfa/tfa_totp and confirm the current password.
2. Scan the QR code, enter the 6-digit code, press "Verify and save" — you are advanced to the "Trust this browser" step.
3. Press "Skip and finish".

Observed: "Setup of ... canceled." and the whole setup is aborted, instead of skipping the trusted-browser step and finishing with TFA (TOTP) enabled.

Expected: "Skip and finish" completes the setup with the already-configured plugin(s) enabled.

Proposed resolution

Remove the '#submit' override (keep '#limit_validation_errors' => []) so the button falls through to the default submitForm() and hits the existing skip branch. Even with limited validation the triggering button value is still preserved under both $values['op'] and $values['skip'] (core Button #name defaults to 'op'; FormValidator::handleErrorsWithLimitedValidation() writes the triggering element's #value back to both #parents and #name), so the strict comparison in the skip branch holds. The separate "Cancel" button on the first/single step keeps ::cancelForm.

$form['actions']['skip'] = [
  '#type' => 'submit',
  '#value' => $count > 0 ? $this->t('Skip') : $this->t('Skip and finish'),
  '#limit_validation_errors' => [],
];

Remaining tasks

• Review the MR.
• Confirm ordering against #3451488 / MR !163 — that fix is a precondition for reaching the step where this button appears.

User interface changes

No layout changes. Behavioural only: at a non-first step, "Skip and finish" now completes setup instead of showing "Setup of ... canceled.".

API changes

None.

Data model changes

None.