Fix false negatives for Rails/ActionControllerFlashBeforeRender when using implicit render or rescue block

[tldn0718]

This PR fixes two false negatives in Rails/ActionControllerFlashBeforeRender. One pertains to implicit render with branch blocks, and the other to rescue blocks.

Branch Block with Implicit Render

Expected Behavior

The cop should register an offense when flash is used within an if block or rescue block, and the action ends without an explicit call to render or redirect_to, resulting in an implicit render.

class HomeController < ActionController::Base
  def create
    flash[:alert] = "msg" if condition
    ^^^^^ Use `flash.now` before `render`.
  end
end

class HomeController < ActionController::Base
  def create
    begin
      do_something
      flash[:alert] = "msg in begin"
      ^^^^^ Use `flash.now` before `render`.
    rescue
      flash[:alert] = "msg in rescue"
      ^^^^^ Use `flash.now` before `render`.
    end
  end
end

Actual Behavior

No offenses are registered.

Rescue Block with Explicit Render

Expected Behavior

The cop should register an offense when flash is used within a begin-rescue block if an explicit render is called outside the block.

class HomeController < ActionController::Base
  def create
    begin
      do_something
      flash[:alert] = "msg in begin"
      ^^^^^ Use `flash.now` before `render`.
    rescue
      flash[:alert] = "msg in rescue"
      ^^^^^ Use `flash.now` before `render`.
    end

    render :index
  end
end

Actual Behavior

No offenses are registered.

Changes

This PR fixes the two types of false negatives and adds extensive testing. The fixes related to implicit render affected existing tests, necessitating the inclusion of the rescue block-related fix in the same PR. Please understand that I could not submit these fixes as separate PRs.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Thanks!