Make search_form_* helpers honour per-search search_key (#1118)

[ryoya1122]

Closes #1118.

Problem

When a search_key: is passed to Model.ransack, the form-building helpers ignore it. The inputs are emitted under the global default key (q[...]) regardless of the override:

q = Person.ransack({}, search_key: :people_search)
search_form_for(q) { |f| f.text_field :name_eq }
# => <input type="text" name="q[name_eq]" id="q_name_eq" />
#                       ^^^ should be people_search[name_eq] / people_search_name_eq

The same applies to search_form_with and turbo_search_form_for. sort_link and sort_url already read the key from the search's context, so the form path is the outlier.

Cause

The two private finalisers used the global default instead of asking the search:

def finalize_form_options(options, html_options)
  options[:as] ||= Ransack.options[:search_key]   # <- global, ignores per-search override
  ...
end

def finalize_form_with_options(options, html_options)
  options[:scope] ||= Ransack.options[:search_key] # <- same
  ...
end

The reporter linked both lines in #1118.

Fix

finalize_form_options / finalize_form_with_options now take the search and read search.context.search_key, the same source sort_link / sort_url already use (see lib/ransack/helpers/form_helper.rb#L200 and #L249).

Context#search_key is initialised as options[:search_key] || Ransack.options[:search_key], so the global default still applies when no per-search override is given.

Compatibility

Model.ransack call	Before	After
Person.ransack(...)	inputs under q[...] (global default)	inputs under q[...] (unchanged)
Person.ransack(...) after Ransack.configure { ... :example }	inputs under example[...] (global)	inputs under example[...] (unchanged)
Person.ransack(..., search_key: :people_search)	inputs under q[...] ❌ (#1118)	inputs under people_search[...] ✓
options[:as] / options[:scope] explicitly set by caller	wins via `

The finalize_form_* methods are private, so the signature change is internal.

Tests

Two new specs in spec/ransack/helpers/form_helper_spec.rb, alongside the existing global-default tests, both red before the fix and green after:

• #search_form_for with per-search search_key -> expects people_search_name_eq
• #search_form_with with per-search search_key -> expects people_search[name_eq]

Full suite: 514 examples, 0 failures, 1 pending on v5.0.0 + this branch (SQLite, ActiveRecord 7.2.3.1, Ruby 3.4.9).

Targets v5.0.0 per #1640.