[iOS] Add Brave Leo deep link. (uplift to 1.64.x) (#22604)

Uplift of #22598 (squashed) to release

ios/brave-ios/Sources/Brave/Frontend/Browser/BrowserViewController/BVC+Callout.swift

@@ -336,4 +336,14 @@ extension BrowserViewController {
       }
     }
   }
+
+  func presentBraveLeoDeepLink() {
+    // If the onboarding has not completed we do not show any promo screens.
+    // This will most likely be the case for users who have not installed the app yet.
+    if Preferences.Onboarding.basicOnboardingCompleted.value != OnboardingState.completed.rawValue {
+      return
+    }
+
+    openBraveLeo()
+  }
 }

ios/brave-ios/Sources/Brave/Frontend/Browser/NavigationRouter.swift

@@ -12,6 +12,7 @@ import BraveWidgetsModels
 // To open a URL use /open-url or to open a blank tab use /open-url with no params
 public enum DeepLink: String {
   case vpnCrossPlatformPromo = "vpn_promo"
+  case braveLeo = "brave_leo"
 }
 
 // The root navigation for the Router. Look at the tests to see a complete URL
@@ -80,6 +81,8 @@ public enum NavigationPath: Equatable {
     switch link {
     case .vpnCrossPlatformPromo:
       bvc.presentVPNInAppEventCallout()
+    case .braveLeo:
+      bvc.presentBraveLeoDeepLink()
     }
   }