REDI-100: adopt MockK and rewrite unit tests to use it

Replace the hand-written CharacterRepository fakes in the ViewModel and
UseCase unit tests with MockK mocks (coEvery / coVerify). This is a
deliberate showcase of MockK and intentionally diverges from the repo's
"prefer fakes over mocks" guidance.

- Add io.mockk:mockk 1.14.3 to the version catalog and the unit-test bundle;
  add it explicitly to DomainModuleConventionPlugin (domain does not consume
  the bundle).
- CharacterListViewModelTest: strict mockk, per-page coEvery stubs; the
  paging/in-flight guards are expressed via coVerify(exactly = ...) and
  coVerifyOrder instead of fake call counters.
- CharacterDetailViewModelTest: relaxed mockk so "missing id" needs no
  stubbing; explicit coEvery elsewhere.
- GetCharactersPageUseCaseTest: mockk + coVerify replaces the inline fake.
- Move character()/characterDetails() fixtures to CharacterFixtures.kt and
  delete FakeCharacterRepository.kt.
- NetworkCharacterRepositoryTest stays on Ktor MockEngine (MockK is for
  Kotlin collaborator interfaces, not the HTTP transport).

gradle/libs.versions.toml

@@ -40,6 +40,7 @@ junitJupiter = "5.11.4"
 junitPlatform = "1.11.4"
 turbine = "1.2.0"
 assertk = "0.28.1"
+mockk = "1.14.3"
 androidxTestExt = "1.3.0"
 androidxTestRunner = "1.7.0"
 androidxEspresso = "3.7.0"
@@ -119,6 +120,7 @@ junit-jupiter-engine = { module = "org.junit.jupiter:junit-jupiter-engine", vers
 junit-platform-launcher = { module = "org.junit.platform:junit-platform-launcher", version.ref = "junitPlatform" }
 turbine = { module = "app.cash.turbine:turbine", version.ref = "turbine" }
 assertk = { module = "com.willowtreeapps.assertk:assertk", version.ref = "assertk" }
+mockk = { module = "io.mockk:mockk", version.ref = "mockk" }
 androidx-test-runner = { module = "androidx.test:runner", version.ref = "androidxTestRunner" }
 androidx-test-ext-junit = { module = "androidx.test.ext:junit", version.ref = "androidxTestExt" }
 androidx-test-espresso-core = { module = "androidx.test.espresso:espresso-core", version.ref = "androidxEspresso" }
@@ -142,7 +144,7 @@ ktor = [
 ]
 lifecycle-compose = ["androidx-lifecycle-runtime-compose", "androidx-lifecycle-viewmodel-compose"]
 views = ["androidx-appcompat", "material", "androidx-recyclerview", "androidx-fragment-ktx"]
-unit-test = ["junit-jupiter-api", "kotlinx-coroutines-test", "turbine", "assertk"]
+unit-test = ["junit-jupiter-api", "kotlinx-coroutines-test", "turbine", "assertk", "mockk"]
 # Instrumented Compose UI test (androidTest): ComposeTestRule + AndroidJUnit4 runner.
 # espresso-core/runner are pinned to current versions: Compose's test rule drives Espresso's
 # onIdle, and the transitive espresso 3.5.0 calls InputManager.getInstance() (removed on API 34+),