fix: preserve merge error context (#3)

Problem
- Merge failures currently collapse 405 and 409 responses to generic ServiceError cases, even when Forgejo returns a useful response body.

Change
- Keeps the existing notMergeable and mergeConflict fallbacks for empty responses.
- Preserves non-empty Forgejo response bodies as ServiceError.httpError for merge failures.
- Extracts the common JSON message field for clearer LocalizedError descriptions.

Tests
- DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test

Co-authored-by: Piotr Durlej <pdurlej@users.noreply.github.com>
Reviewed-on: https://codeberg.org/secana/ForgejoKit/pulls/3

Sources/ForgejoKit/Services/ForgejoClient.swift

@@ -488,7 +488,11 @@ public enum ServiceError: LocalizedError, Sendable {
         case .invalidResponse:
             "Invalid response from server"
         case let .httpError(statusCode, message):
-            message.isEmpty ? "HTTP error: \(statusCode)" : "HTTP \(statusCode): \(message)"
+            if let message = Self.normalizedHTTPMessage(message) {
+                "HTTP \(statusCode): \(message)"
+            } else {
+                "HTTP error: \(statusCode)"
+            }
         case .notMergeable:
             "This pull request cannot be merged"
         case .mergeConflict:
@@ -497,4 +501,20 @@ public enum ServiceError: LocalizedError, Sendable {
             "Decoding failed: \(detail)"
         }
     }
+
+    private static func normalizedHTTPMessage(_ message: String) -> String? {
+        let trimmed = message.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !trimmed.isEmpty else {
+            return nil
+        }
+        guard
+            let data = trimmed.data(using: .utf8),
+            let object = try? JSONSerialization.jsonObject(with: data) as? [String: Any],
+            let jsonMessage = object["message"] as? String
+        else {
+            return trimmed
+        }
+        let trimmedJSONMessage = jsonMessage.trimmingCharacters(in: .whitespacesAndNewlines)
+        return trimmedJSONMessage.isEmpty ? trimmed : trimmedJSONMessage
+    }
 }

Sources/ForgejoKit/Services/PullRequestService.swift

@@ -173,11 +173,11 @@ public final class PullRequestService: Sendable {
         do {
             _ = try await client.performRequestNoContent(url: url, method: "POST", body: jsonData, validateStatus: true)
         } catch let error as ServiceError {
-            if case let .httpError(statusCode, _) = error {
+            if case let .httpError(statusCode, message) = error {
                 switch statusCode {
-                case 405:
+                case 405 where message.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty:
                     throw ServiceError.notMergeable
-                case 409:
+                case 409 where message.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty:
                     throw ServiceError.mergeConflict
                 default:
                     throw error

Tests/ForgejoKitTests/NotificationTests.swift

@@ -75,6 +75,21 @@ struct NotificationTests {
         #expect(ServiceError.decodingFailed(detail: "missing key").errorDescription == "Decoding failed: missing key")
     }
 
+    @Test func httpErrorDescriptionUsesForgejoMessageBody() {
+        let error = ServiceError.httpError(
+            statusCode: 405,
+            message: #"{"message":"This pull request has failing status checks"}"#,
+        )
+
+        #expect(error.errorDescription == "HTTP 405: This pull request has failing status checks")
+    }
+
+    @Test func httpErrorDescriptionFallsBackToRawBody() {
+        let error = ServiceError.httpError(statusCode: 500, message: "database unavailable")
+
+        #expect(error.errorDescription == "HTTP 500: database unavailable")
+    }
+
     // MARK: - Full API response decoding
 
     @Test func decodesNotificationArray() throws {