phpstan fails on SettingsForm and on Drupal 11 symbols in the previous major job

Part of #3596034 (closed).

Problem/motivation

Both phpstan jobs fail on 1.x. On Drupal 11, SettingsForm calls \Drupal::config() and \Drupal::messenger() although ConfigFormBase already provides both. On Drupal 10 the other 19 errors are symbols that exist only on Drupal 11 or PHPUnit 10: RequirementSeverity, the Hook and LegacyHook attributes, ThemeSettingsProvider and RunTestsInSeparateProcesses. The module already guards them with DeprecationHelper or uses them as attributes, which older versions ignore.

Proposed resolution

Use $this->config() and $this->messenger() in SettingsForm. Add a phpstan.neon that copies the template default and ignores those five symbols by exact message, without a baseline.

AI Compliance

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