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/javascript/dashboard/store/modules/conversationUnreadCounts.js

@@ -48,9 +48,6 @@ export const actions = {
       // Ignore errors so the sidebar can continue rendering without badges.
     }
   },
-  clear({ commit }) {
-    commit(types.SET_CONVERSATION_UNREAD_COUNTS, {});
-  },
 };
 
 export const mutations = {

app/javascript/dashboard/store/modules/specs/conversationUnreadCounts/actions.spec.js

@@ -39,15 +39,4 @@ describe('#actions', () => {
       expect(commit).not.toHaveBeenCalled();
     });
   });
-
-  describe('#clear', () => {
-    it('clears unread counts', () => {
-      actions.clear({ commit });
-
-      expect(commit).toHaveBeenCalledWith(
-        types.SET_CONVERSATION_UNREAD_COUNTS,
-        {}
-      );
-    });
-  });
 });