test: add comprehensive unit test suite (~85 new tests)

Add unit tests across all model and server infrastructure layers,
increasing test count from 7 to 92. Covers serde round-trips, enum
methods, defaults, config parsing, error mapping, PKCE crypto (with
RFC 7636 test vector), OAuth store, and SearXNG ranking/dedup logic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/models/provider.rs

@@ -72,3 +72,84 @@ pub struct ProviderConfig {
     pub selected_embedding: String,
     pub api_key_set: bool,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn llm_provider_label_ollama() {
+        assert_eq!(LlmProvider::Ollama.label(), "Ollama");
+    }
+
+    #[test]
+    fn llm_provider_label_hugging_face() {
+        assert_eq!(LlmProvider::HuggingFace.label(), "Hugging Face");
+    }
+
+    #[test]
+    fn llm_provider_label_openai() {
+        assert_eq!(LlmProvider::OpenAi.label(), "OpenAI");
+    }
+
+    #[test]
+    fn llm_provider_label_anthropic() {
+        assert_eq!(LlmProvider::Anthropic.label(), "Anthropic");
+    }
+
+    #[test]
+    fn llm_provider_serde_round_trip() {
+        for variant in [
+            LlmProvider::Ollama,
+            LlmProvider::HuggingFace,
+            LlmProvider::OpenAi,
+            LlmProvider::Anthropic,
+        ] {
+            let json = serde_json::to_string(&variant)
+                .unwrap_or_else(|_| panic!("serialize {:?}", variant));
+            let back: LlmProvider =
+                serde_json::from_str(&json).unwrap_or_else(|_| panic!("deserialize {:?}", variant));
+            assert_eq!(variant, back);
+        }
+    }
+
+    #[test]
+    fn model_entry_serde_round_trip() {
+        let entry = ModelEntry {
+            id: "llama3.1:8b".into(),
+            name: "Llama 3.1 8B".into(),
+            provider: LlmProvider::Ollama,
+            context_window: 8192,
+        };
+        let json = serde_json::to_string(&entry).expect("serialize ModelEntry");
+        let back: ModelEntry = serde_json::from_str(&json).expect("deserialize ModelEntry");
+        assert_eq!(entry, back);
+    }
+
+    #[test]
+    fn embedding_entry_serde_round_trip() {
+        let entry = EmbeddingEntry {
+            id: "nomic-embed".into(),
+            name: "Nomic Embed".into(),
+            provider: LlmProvider::HuggingFace,
+            dimensions: 768,
+        };
+        let json = serde_json::to_string(&entry).expect("serialize EmbeddingEntry");
+        let back: EmbeddingEntry = serde_json::from_str(&json).expect("deserialize EmbeddingEntry");
+        assert_eq!(entry, back);
+    }
+
+    #[test]
+    fn provider_config_serde_round_trip() {
+        let cfg = ProviderConfig {
+            provider: LlmProvider::Anthropic,
+            selected_model: "claude-3".into(),
+            selected_embedding: "embed-v1".into(),
+            api_key_set: true,
+        };
+        let json = serde_json::to_string(&cfg).expect("serialize ProviderConfig");
+        let back: ProviderConfig = serde_json::from_str(&json).expect("deserialize ProviderConfig");
+        assert_eq!(cfg, back);
+    }
+}