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/infrastructure/error.rs

@@ -41,3 +41,53 @@ impl IntoResponse for Error {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use axum::response::IntoResponse;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn state_error_display() {
+        let err = Error::StateError("bad state".into());
+        assert_eq!(err.to_string(), "bad state");
+    }
+
+    #[test]
+    fn database_error_display() {
+        let err = Error::DatabaseError("connection lost".into());
+        assert_eq!(err.to_string(), "database error: connection lost");
+    }
+
+    #[test]
+    fn config_error_display() {
+        let err = Error::ConfigError("missing var".into());
+        assert_eq!(err.to_string(), "configuration error: missing var");
+    }
+
+    #[test]
+    fn state_error_into_response_500() {
+        let resp = Error::StateError("oops".into()).into_response();
+        assert_eq!(resp.status(), StatusCode::INTERNAL_SERVER_ERROR);
+    }
+
+    #[test]
+    fn database_error_into_response_503() {
+        let resp = Error::DatabaseError("down".into()).into_response();
+        assert_eq!(resp.status(), StatusCode::SERVICE_UNAVAILABLE);
+    }
+
+    #[test]
+    fn config_error_into_response_500() {
+        let resp = Error::ConfigError("bad cfg".into()).into_response();
+        assert_eq!(resp.status(), StatusCode::INTERNAL_SERVER_ERROR);
+    }
+
+    #[test]
+    fn io_error_into_response_500() {
+        let io_err = std::io::Error::new(std::io::ErrorKind::NotFound, "not found");
+        let resp = Error::IoError(io_err).into_response();
+        assert_eq!(resp.status(), StatusCode::INTERNAL_SERVER_ERROR);
+    }
+}