chore: add must_use to 27 non-Result public functions

- constructors: KnowledgeEntity, TextChunk, Scratchpad, IngestionTask,
  Conversation, KnowledgeRelationship, Message, TextContent,
  KnowledgeEntityEmbedding, TextChunkEmbedding
- accessors: Theme::as_str, Theme::initial_theme, TaskState::as_str,
  TaskState::display_label, StorageManager::backend_kind,
  StorageManager::local_base_path, EmbeddingProvider::backend_label,
  EmbeddingProvider::dimension, EmbeddingProvider::model_code
- queries: TaskState::is_terminal, IngestionTask::can_retry,
  KnowledgeEntityType::variants, StorageManager::resolve_local_path,
  resolve_base_dir, IngestionTask::lease_duration
- helpers: Message::format_history
- builders: StorageManager::with_backend

common/src/utils/embedding.rs

@@ -78,6 +78,7 @@ enum EmbeddingInner {
 }
 
 impl EmbeddingProvider {
+    #[must_use]
     pub fn backend_label(&self) -> &'static str {
         match self.inner {
             EmbeddingInner::Hashed { .. } => "hashed",
@@ -86,6 +87,7 @@ impl EmbeddingProvider {
         }
     }
 
+    #[must_use]
     pub fn dimension(&self) -> usize {
         match &self.inner {
             EmbeddingInner::Hashed { dimension } | EmbeddingInner::FastEmbed { dimension, .. } => {
@@ -95,6 +97,7 @@ impl EmbeddingProvider {
         }
     }
 
+    #[must_use]
     pub fn model_code(&self) -> Option<String> {
         match &self.inner {
             EmbeddingInner::FastEmbed { model_name, .. } => Some(model_name.to_string()),