Issue #3614968: Configure and resolve MR CI Pipelines

• Adds cspell, PHPStan, PHPCS, ESLint, Stylelint to MR pipelines
  • Does not allow failures
  • Sets PHPStan strictness to level 6
• Makes necessary changes to allow CI jobs to pass
• Adds PHPUnit regression coverage for:
  • Block theme suggestion generation and deduplication.
  • The legacy search form alter hook bridge.
  • radix_clean_identifier() handling of strings, NULL, non-string values and special characters.

AI Disclosure: Some of this PR was implemented with the help of an LLM. All code has been human-tested and verified.

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Closes #3614968