fix: address final review findings for provider migration

- Add SessionCoordinator regression test covering switch from an
  authenticated ABox profile to Local experience: remote session is
  logged out and cleared, only Local is the active/observable session,
  credentials are cleared, auto-login disabled, and gateway state stays
  isolated (verified to fail when switch cleanup is removed).
- Replace if/else provider dispatch with exhaustive when over
  ProviderType in all three routing gateways so future enum additions
  fail compilation.
- Silence the known ExperimentalCoroutinesApi opt-in warning in
  ProviderSelectionViewModelTest with a class-level @OptIn.

testing/fakes/src/test/kotlin/com/flagship/abox/manager/testing/fakes/SessionCoordinatorTest.kt

@@ -153,11 +153,47 @@ class SessionCoordinatorTest {
         assertTrue(!fixture.profiles.loginPreferences(remoteProfile.id).autoLoginEnabled)
     }
 
+    @Test
+    fun switchingFromAuthenticatedAboxToLocalClearsRemoteSessionAndIsolatesState() = runTest {
+        val fixture = Fixture()
+        fixture.coordinator.selectProvider(remoteProfile)
+        val loginResult = fixture.coordinator.login(
+            remoteProfile.id,
+            credentials,
+            rememberUsername = true,
+            autoLogin = true,
+        )
+
+        val switchResult = fixture.coordinator.selectProvider(localProfile)
+
+        assertTrue(loginResult is DomainResult.Success)
+        assertTrue(switchResult is DomainResult.Success)
+        assertEquals(
+            localProfile.id,
+            (switchResult as DomainResult.Success).value?.profileId,
+        )
+        assertEquals(localProfile.id, fixture.profiles.activeProfile()?.id)
+        assertEquals(
+            localProfile.id,
+            fixture.coordinator.observeActiveSession().first()?.profileId,
+        )
+        assertNull(fixture.remote.observeSession(remoteProfile.id).first())
+        assertNull(fixture.remote.observeSession(localProfile.id).first())
+        assertEquals(
+            localProfile.id,
+            fixture.local.observeSession(localProfile.id).first()?.profileId,
+        )
+        assertNull(
+            (fixture.credentials.loadCredentials(remoteProfile.id) as DomainResult.Success).value,
+        )
+        assertTrue(!fixture.profiles.loginPreferences(remoteProfile.id).autoLoginEnabled)
+    }
+
     private class Fixture {
         val profiles = FakeProviderProfileStore()
         val credentials = FakeCredentialStore()
         val remote = FakeSessionGateway()
-        private val local = FakeLocalSessionGateway()
+        val local = FakeLocalSessionGateway()
         val coordinator = SessionCoordinator(profiles, credentials, remote, local)
     }