feat(conversations): remove unread count feature flag (CW-7237) (#14610)

## Description

Make conversation unread counts always available at runtime by removing
account feature checks from the API endpoint, unread-count listener,
notifier, and ActionCable broadcast path.

Update the dashboard to fetch sidebar unread counts for the active
account without checking FEATURE_FLAGS.CONVERSATION_UNREAD_COUNTS, and
remove the now-unused store clear action that only supported the
disabled state.

Keep the feature entry in config/features.yml to preserve flag bit
order, but mark it enabled and deprecated so fresh installs default to
the always-on behavior while feature-management UI hides it.

Leave existing installation default rows untouched; no migration is
included, so upgraded installs may still store the old flag value but
runtime behavior no longer depends on it.

Update specs around the new always-on contract and remove obsolete
disabled-feature assertions.

Fixes # CW-7237

## Type of change

Please delete options that are not relevant.

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

Update specs around the new always-on contract and remove obsolete
disabled-feature assertions. Ran specs locally for the changes.


## Checklist:

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

app/controllers/api/v1/accounts/conversations/unread_counts_controller.rb

@@ -1,16 +1,6 @@
 class Api::V1::Accounts::Conversations::UnreadCountsController < Api::V1::Accounts::BaseController
-  before_action :ensure_unread_counts_enabled
-
   def index
     counts = ::Conversations::UnreadCounts::Counter.new(account: Current.account, user: Current.user).perform
     render json: { payload: counts }
   end
-
-  private
-
-  def ensure_unread_counts_enabled
-    return if Current.account.feature_enabled?('conversation_unread_counts')
-
-    render json: { error: I18n.t('errors.conversations.unread_counts.feature_not_enabled') }, status: :forbidden
-  end
 end