fix: Stabilize help center builder error spec (#14725)

## Description

Stabilizes the enterprise help center article builder source URL
validation spec by asserting the custom exception via its class name and
message text. This keeps the spec focused on the intended behavior while
avoiding brittle custom exception constant identity checks in
CI/reloading environments.

A bunch of builds on different PRs have been failing because of this
error, sample traces are below:
*
https://app.circleci.com/pipelines/github/chatwoot/chatwoot/114064/workflows/bdb6eca9-3b65-4c38-b8cf-f2f8564476f8/jobs/158777
*
https://app.circleci.com/pipelines/github/chatwoot/chatwoot/114064/workflows/bdb6eca9-3b65-4c38-b8cf-f2f8564476f8/jobs/158777

Fixes # N/A

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

- `/Users/sonymathew/.rbenv/shims/bundle exec rspec
spec/enterprise/services/onboarding/help_center_article_builder_spec.rb`
- `/Users/sonymathew/.rbenv/shims/bundle exec rubocop
spec/enterprise/services/onboarding/help_center_article_builder_spec.rb`
- `git diff --check`

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] My changes generate no new warnings
- [x] New and existing unit tests pass locally with my changes
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] Any dependent changes have been merged and published in downstream
modules

spec/enterprise/services/onboarding/help_center_article_builder_spec.rb

@@ -12,7 +12,10 @@ RSpec.describe Onboarding::HelpCenterArticleBuilder do
 
       expect(Firecrawl::Configuration).not_to receive(:client)
       expect { builder.perform }
-        .to raise_error(Onboarding::HelpCenterErrors::ArticleBuildFailed, /no source urls/)
+        .to raise_error(StandardError) { |error|
+          expect(error.class.name).to eq('Onboarding::HelpCenterErrors::ArticleBuildFailed')
+          expect(error.message).to include('no source urls')
+        }
     end
   end
 end