5563 - Replace helper spec stubs with real tests

[fuentesjr]

What github issue is this PR for, if any?

Part of #5563, intentionally not Resolves — 16 stub specs remain after this.

What changed, and why?

Replaces five pending "add some tests for X" stubs under spec/helpers/ with
real specs (one commit per file), following existing helper-spec style
(mileage_rates_helper_spec.rb, court_dates_helper_spec.rb):

• DateHelper — the substantive one: validate_date (parses valid parts,
  raises Date::Error on any blank part) and parse_date (the Rails
  multiparameter (1i)/(2i)/(3i) happy path, the invalid-date path that adds a
  model error and leaves the field unset, and the all-blank case that strips the
  date-part keys and adds no date)
• TemplateHelper — tag sanitize/validate helpers and active_if /
  active_if_status
• AllCasaAdmins::CasaOrgsHelper — selected_organization returns the
  assigned @casa_org (and nil when unassigned)
• RequestHeaderHelper / ApiBaseHelper — constant-value pins; these modules
  define only constants, so a value assertion is the available real coverage

No production code changed.

How is this tested? (please write rspec and jest tests!) 💖💪

• bundle exec rspec spec/helpers/{request_header,template,date,api_base}_helper_spec.rb spec/helpers/all_casa_admins/casa_orgs_helper_spec.rb
  → 26 examples, 0 failures
• bundle exec standardrb on the five files → no offenses
• No pending / skip / xit remains in any of them

Screenshots please :)

N/A — test-only change, no UI.

Feelings gif (optional)

N/A