Add application_form_class method to Flex::Case (#188)

## Changes

- Added `application_form_class` class method to `Flex::Case` that
returns the application form class name using string substitution
- Updated `start_on_application_form_created` in
`business_process_builder.rb` to use the new method instead of hardcoded
string manipulation
- Allows subclasses to override the method for custom application form
naming conventions

This changes allows us to have application form subclasses that aren't suffixed with ApplicationForm, such as PaidLeaveRequest or ActivityReport


## Context

This change refactors hardcoded string substitution logic into a
reusable class method. Previously, the business process builder was
directly performing `case_class.name.sub("Case",
"ApplicationFormCreated")` to generate event names. Now it calls
`case_class.application_form_class` which returns the application form
class name that can be customized by subclasses.

The implementation maintains backward compatibility by using the same
string substitution logic (`name.sub("Case", "ApplicationForm")`) as the
default behavior.

**Key areas for review:**
- Verify the string substitution logic works correctly for all Case
subclasses
- Confirm that event name generation still matches what ApplicationForm
classes publish
- Check that the method allows appropriate subclass customization

## Testing

- **Full test suite**: All 787 tests pass with 0 failures
- **Linting**: All style checks pass with no offenses detected  
- **Event name verification**: Confirmed in Rails console that event
names match correctly:
  - `PassportCase.application_form_class` → `"PassportApplicationForm"`
  - `TestCase.application_form_class` → `"TestApplicationForm"`
- **Business process functionality**: PassportBusinessProcess acceptance
test passes, confirming cases are created correctly when application
forms are submitted

The existing test suite provides good coverage for this change since it
exercises the business process event handling that relies on the
modified code.

---

**Link to Devin run**:
https://app.devin.ai/sessions/353c6623acf94683970eea40c0e23270
**Requested by**: @lorenyu

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: Loren Yu <yuloren@gmail.com>

Author: devin-ai-integration[bot]

app/models/flex/business_process_builder.rb

@@ -53,7 +53,7 @@ def start(step_name, on: nil, &handler)
       end
 
       def start_on_application_form_created(step_name)
-        event_name = case_class.name.sub("Case", "ApplicationFormCreated")
+        event_name = "#{case_class.application_form_class}Created"
         start(step_name, on: event_name) do |event|
           case_class.new(application_form_id: event[:payload][:application_form_id])
         end

app/models/flex/case.rb

@@ -30,6 +30,16 @@ def self.base_attributes_for_generator
         "facts:jsonb"
       ]
     end
+
+    # Returns the application form class name for this case class.
+    # Subclasses can override this method to customize the application form class naming.
+    #
+    # @return [String] The application form class name
+    # @example
+    #   PassportCase.application_form_class #=> "PassportApplicationForm"
+    def self.application_form_class
+      name.sub("Case", "ApplicationForm")
+    end
     attribute :application_form_id, :uuid
 
     attribute :status, :integer, default: 0