Add unit tests for all types and error handling

54 tests covering serialization/deserialization round-trips, builder
patterns, skip_serializing_if behavior, untagged enums, field renames,
and error type conversions.

Test modules added to:
- types/common.rs: Think, Stop, Options, OptionsBuilder, ModelDetails
- types/generate.rs: GenerateRequest, GenerateResponse, builder
- types/chat.rs: Role, Message, ChatRequest, Tool, ChatResponse
- types/pull.rs: PullRequest, PullResponse, builder
- types/version.rs: VersionResponse
- types/tags.rs: TagsResponse
- types/ps.rs: PsResponse
- error.rs: Display formatting, From conversions, Error trait

src/types/pull.rs

@@ -45,3 +45,51 @@ impl PullRequestBuilder {
 pub struct PullResponse {
     pub status: String,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn builder_minimal() {
+        let request = PullRequest::builder("llama3").build();
+        assert_eq!(request.model, "llama3");
+        assert!(request.insecure.is_none());
+        assert!(request.stream.is_none());
+    }
+
+    #[test]
+    fn builder_with_options() {
+        let request = PullRequest::builder("llama3")
+            .stream(true)
+            .insecure(false)
+            .build();
+        assert_eq!(request.stream, Some(true));
+        assert_eq!(request.insecure, Some(false));
+    }
+
+    #[test]
+    fn request_skips_none_fields() {
+        let request = PullRequest::builder("llama3").build();
+        let json = serde_json::to_value(&request).unwrap();
+        let obj = json.as_object().unwrap();
+        assert!(obj.contains_key("model"));
+        assert!(!obj.contains_key("insecure"));
+        assert!(!obj.contains_key("stream"));
+    }
+
+    #[test]
+    fn request_includes_set_fields() {
+        let request = PullRequest::builder("llama3").stream(true).build();
+        let json = serde_json::to_value(&request).unwrap();
+        assert_eq!(json, json!({"model": "llama3", "stream": true}));
+    }
+
+    #[test]
+    fn response_deserialize() {
+        let json = json!({"status": "pulling manifest"});
+        let response: PullResponse = serde_json::from_value(json).unwrap();
+        assert_eq!(response.status, "pulling manifest");
+    }
+}