[CST] Shifts Claims Letters doctype logic into downloader

[iandonovan]

Summary

This is an iterative change to #19241.

That original work there added the claims letters to the controller that is hit by vets-website, and this change moves everything one step upstream: the logic to determine which letter doctypes to retrieve is now located in the ClaimLetterDownloader. Prior to this change, the Website was the only client to use the feature flags to determine which letters to get. The Virtual Agent and Mobile were both configured to ignore those flags and only ever get doctype 184.

I did this after consulting with the Virtual Agent team and with the mobile team. They agree that the delineation of doctypes between clients is not necessary, and that all letters can be shown to everyone.

There is a separate feature flipper called mobile_filter_doc_27_decision_letters_out that now seems redundant. It exists to, even if the feature flipper enabling doctype 27 is turned on in general, strip those letters from the mobile app. We do not know why this exists. (The flipper enabling doctype 27 will remain off, as the Board has not approved digital delivery of these letters.)

The testing for the Claims Letters Controller is here, in the aforementioned PR.

• This work is behind a feature toggle (flipper): YES
• I'm on BMT2.
• I am not introducing a new flipper, but in order for the additional claim letters to be rendered, we will have to enable the following two flippers:

1. cst_include_ddl_5103_letters
2. cst_include_ddl_sqd_letters

Related issue(s)

• Original change request
• Investigation
• Issue for the actions to be taken

Testing done

• New code is covered by unit tests
• The Claim Letter downloader did not include a human-readable name with each letter. As a result, the client side was responsible for showing one (if at all). For example, this is the transformation done on vets-website.

What areas of the site does it impact?

This affects the Claim Letters view.

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

[github-actions]

Error: A file (or its parent directories) does not have a CODEOWNERS entry. Please update the .github/CODEOWNERS file and add the entry for the Offending file: app/controllers/v0/virtual_agent_claim_letters_controller.rb

[github-actions]

Error: A file (or its parent directories) does not have a CODEOWNERS entry. Please update the .github/CODEOWNERS file and add the entry for the Offending file: spec/controllers/v0/virtual_agent_claim_letters_controller_spec.rb

[rjohnson2011]

I know these were already like this, but we have new conventions for Flipper specs. Do you mind updating these in accordance please?
https://depo-platform-documentation.scrollhelp.site/developer-docs/feature-toggles-guide#FeatureTogglesGuide-Backendexample
Particularly this part:

      # Recommended to limit the feature flag state to the scope of this test
      allow(Flipper).to receive(:enabled?).with(:feature_flag).and_return(true)
      # Not recommended. Affects global state and can have unintended side effects when tests run in parallel
      Flipper.enable(:feature_flag)