Handle TopicUiState.Error without crashing TopicScreen

[saadkhalidkhan]

Summary

• Replace TODO() in TopicScreen with error UI when topic loading fails
• Show a user-visible message for news loading errors instead of a hardcoded placeholder
• Add a compose test covering TopicUiState.Error
• Fix scrollbar item count to include toolbar and header for news loading/error states

Fixes #2108

Test plan

• ./gradlew assembleDemoDebug passes
• ./gradlew :feature:topic:impl:compileDemoDebugAndroidTestKotlin passes
• Run TopicScreenTest.topicError_whenTopicIsError_isShown on device/emulator

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[gemini-code-assist]

Code Review

This pull request implements the error state for the topic screen by adding dedicated UI components, localized error strings, and a corresponding UI test. The feedback highlights a potential issue in the topicItemsSize function where the success state logic may not accurately account for all items, which could lead to incorrect scrollbar behavior.