Expose Chef's template helper variables when rendering

[tas50]

Fixes chefspec#795

Problem

Chef's template provider sets helper variables on the template context before rendering, in Chef::Provider::Template::Content#file_for_provider:

context[:cookbook_name] = new_resource.cookbook_name unless context.keys.include?(:cookbook_name)
context[:recipe_name] = new_resource.recipe_name unless context.keys.include?(:recipe_name)
context[:recipe_line_string] = new_resource.source_line unless context.keys.include?(:recipe_line_string)
context[:recipe_path] = new_resource.source_line_file unless context.keys.include?(:recipe_path)
context[:recipe_line] = new_resource.source_line_number unless context.keys.include?(:recipe_line)
context[:template_name] = new_resource.source unless context.keys.include?(:template_name)
context[:template_path] = template_location unless context.keys.include?(:template_path)

ChefSpec's renderer only sets node and template_finder, so a template containing <%= cookbook_name %> renders an empty string under test while working correctly during a real Chef run:

expected Chef run to render "/tmp/cbname" matching:
I am from the repro cookbook
but got:
I am from the  cookbook

Fix

Set the same helper variables Chef does, keeping Chef's rule that variables supplied on the resource take precedence.

Testing

Adds a template referencing cookbook_name, template_name and recipe_name to the render_file acceptance example.

• With the fix: 56 examples, 0 failures
• With the fix reverted: 56 examples, 3 failures
• Unit suite: 197 examples, 0 failures

The resource double in spec/unit/renderer_spec.rb is extended with the attributes the renderer now reads.

[tas50]

CI on this PR is red for an unrelated reason: main's bundle cannot be installed on Linux, which breaks every job before any test runs. That is fixed separately in #56, which is green across Ruby 3.1 to 3.4. This PR should go green once #56 lands and this branch is rebased.

Locally, against a working bundle, this branch passes rake unit and the acceptance examples.