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).

build-logic/convention/src/main/kotlin/com/example/architecture/convention/DomainModuleConventionPlugin.kt

@@ -20,6 +20,8 @@ class DomainModuleConventionPlugin : Plugin<Project> {
             add("implementation", libs.findLibrary("kotlinx-coroutines-core").get())
             add("testImplementation", libs.findLibrary("junit-jupiter-api").get())
             add("testImplementation", libs.findLibrary("assertk").get())
+            // Domain doesn't consume the `unit-test` bundle, so MockK is added explicitly here.
+            add("testImplementation", libs.findLibrary("mockk").get())
             add("testRuntimeOnly", libs.findLibrary("junit-jupiter-engine").get())
             // Gradle 9 dropped the bundled launcher; JUnit 5 won't start without it.
             add("testRuntimeOnly", libs.findLibrary("junit-platform-launcher").get())