[management] Add option to disable default all-to-all policy (#3970)

This PR introduces a new configuration option `DisableDefaultPolicy` that prevents the creation of the default all-to-all policy when new accounts are created. This is useful for automation scenarios where explicit policies are preferred.
### Key Changes:
- Added DisableDefaultPolicy flag to the management server config
- Modified account creation logic to respect this flag
- Updated all test cases to explicitly pass the flag (defaulting to false to maintain backward compatibility)
- Propagated the flag through the account manager initialization chain

### Testing:

- Verified default behavior remains unchanged when flag is false
- Confirmed no default policy is created when flag is true
- All existing tests pass with the new parameter

management/server/posture_checks_test.go

@@ -106,7 +106,7 @@ func initTestPostureChecksAccount(am *DefaultAccountManager) (*types.Account, er
 		Role: types.UserRoleUser,
 	}
 
-	account := newAccountWithId(context.Background(), accountID, groupAdminUserID, domain)
+	account := newAccountWithId(context.Background(), accountID, groupAdminUserID, domain, false)
 	account.Users[admin.Id] = admin
 	account.Users[user.Id] = user