fix: render error UI for TopicUiState.Error instead of crashing

[Noahs212]

Summary

• Replace TopicUiState.Error -> TODO() (TopicScreen.kt:132) with a localized error message. TODO() throws NotImplementedError at runtime, so the topic detail screen currently crashes whenever the upstream topic flow emits an error (Result.Error from asResult).
• Add feature_topic_api_error_header string and a private TopicErrorBody composable modeled on SearchNotReadyBody.
• Update topicItemsSize so the scrollbar item count matches the rendered error item (0 -> 1).
• Add errorMessage_whenTopicIsError_isShown to TopicScreenTest. This test fails on main with NotImplementedError and passes after the fix.

The other two TODOs in the same file (hardcoded "Loading news" at L249 and Text("Error") at L253) are in the NewsUiState branch and intentionally out of scope to keep this diff small.

Fixes #2108

Test plan

• ./gradlew :feature:topic:impl:connectedDemoDebugAndroidTest --tests "*TopicScreenTest*"
• ./gradlew :feature:topic:impl:testDemoDebugUnitTest
• ./gradlew spotlessApply
• ./gradlew :feature:topic:impl:lintDemoDebug :feature:topic:api:lintDemoDebug
• Manually verify on a device: force a topic-flow error and confirm the error text appears instead of a crash.

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[gemini-code-assist]

Code Review

This pull request implements the error state for the Topic screen by adding a new error message string, a TopicErrorBody composable, and updating the UI logic to handle TopicUiState.Error. It also includes a new UI test to verify the error message display. Feedback highlights that the topicItemsSize calculation for the success state may be incomplete, potentially affecting scrollbar accuracy, and suggests adding a modifier parameter to the TopicErrorBody composable to follow Compose best practices.

[gemini-code-assist]

While updating this to 1 is correct for the new error state, the existing logic for TopicUiState.Success (lines 185-186) appears to be incorrect as it doesn't account for the Toolbar and Header items which are always rendered in that state. This discrepancy will cause the scrollbar to be incorrectly sized or hidden when news is loading or has an error. Consider updating the entire function to ensure the scrollbar accurately reflects the number of items in the LazyColumn.

[gemini-code-assist]

It is a best practice in Compose to have a modifier parameter for all composables to allow for layout adjustments from the caller. This also ensures that the internal Column uses the passed-in modifier as its base.

Suggested change

	private fun TopicErrorBody() {
	Column(
	horizontalAlignment = Alignment.CenterHorizontally,
	modifier = Modifier
	private fun TopicErrorBody(modifier: Modifier = Modifier) {
	Column(
	horizontalAlignment = Alignment.CenterHorizontally,
	modifier = modifier