HOLD: Extract BaseRegistrationFormBuilder to share fields across form types

[maebeale]

What is the goal of this PR and why is this important?

• Three form builder services (Short, Extended, Scholarship) duplicated identical helper methods, scholarship fields, and contact fields
• Changing a shared field (e.g., adding email confirmation) required updating multiple files independently, risking drift
• Several field key mismatches between builders caused silent bugs in validation and tag assignment

How did you approach the change?

• Extracted BaseRegistrationFormBuilder with shared helpers (add_header, add_field) and reusable field sections (build_basic_contact_fields, build_scholarship_fields, build_consent_fields)
• All three builders now inherit from the base class, eliminating ~210 lines of duplication
• Fixed three bugs discovered during the refactor:
  • Email confirmation key mismatch: Extended form used primary_email_confirmation but controller validation checked confirm_email — confirmation match validation was silently skipped for extended forms
  • workshop_settings vs workshop_environments: Builder created workshop_settings but the service and helper looked for workshop_environments — professional tags were never assigned
  • Redundant confirm email storage: save_form_fields skipped primary_email_confirmation but not confirm_email — short form stored confirmation email unnecessarily
• Updated email row logic in new.html.erb to support 3-column layout (email | confirm | type) when all three fields exist
• Added specs for Short and Extended form builders

UI Testing Checklist

• Short form registration renders correctly with contact, consent, qualitative, and scholarship sections
• Extended form registration renders correctly with all sections
• Email confirmation validation works on both form types
• Email row displays as 3-column on extended form (email | confirm email | email type)
• Email row displays as 2-column on short form (email | confirm email)
• Registration show page displays correctly, skipping confirm email field
• Scholarship application form still builds correctly

Anything else to add?

• This is forward-only: existing forms in the database are untouched. Only newly built forms will use the shared definitions.
• The base class is designed for future extensibility — an AuthenticatedEventRegistrationFormBuilder can inherit the same shared sections and add its own fields.

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