chore(hub): clean up legacy Captain hub flow (#13640)

## Summary
This PR cleans up legacy Hub/Captain integration paths and simplifies
hub URL behavior coverage in tests.

## Changes
- remove legacy Captain account endpoint flow from `ChatwootHub`
- remove obsolete spec coverage tied to that retired flow
- keep hub URL handling centralized in `base_url` with enterprise
overlay precedence
- simplify hub URL specs to assert explicit static URL expectations
where applicable

## Reproduce
Run the focused hub specs from this branch:
- `bundle exec rspec spec/lib/chatwoot_hub_spec.rb
spec/enterprise/lib/chatwoot_hub_spec.rb`

## Testing
Validated locally with:
- `bundle exec rspec spec/lib/chatwoot_hub_spec.rb
spec/enterprise/lib/chatwoot_hub_spec.rb`
- `bundle exec rubocop lib/chatwoot_hub.rb spec/lib/chatwoot_hub_spec.rb
enterprise/lib/enterprise/chatwoot_hub.rb
spec/enterprise/lib/chatwoot_hub_spec.rb`

spec/enterprise/lib/chatwoot_hub_spec.rb

@@ -0,0 +1,21 @@
+require 'rails_helper'
+
+RSpec.describe ChatwootHub do
+  describe '.base_url' do
+    it 'uses the static hub url outside development for enterprise edition' do
+      with_modified_env CHATWOOT_HUB_URL: 'https://custom.example.com' do
+        allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('production'))
+
+        expect(described_class.base_url).to eq('https://hub.2.chatwoot.com')
+      end
+    end
+
+    it 'uses CHATWOOT_HUB_URL in development for enterprise edition' do
+      with_modified_env CHATWOOT_HUB_URL: 'https://custom.example.com' do
+        allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('development'))
+
+        expect(described_class.base_url).to eq('https://custom.example.com')
+      end
+    end
+  end
+end