Conform RuntimeError to HTTPResponseConvertible (#135)

### Motivation


apple/swift-openapi-generator#609 (comment)

### Modifications

Confirm `RuntimeError` to `HTTPResponseConvertible` and provide granular
status codes.

### Result

Response codes for bad user input will be 4xx (instead of 500)

### Test Plan

Unit tests.

---------

Co-authored-by: Gayathri Sairamkrishnan <[email protected]>

Sources/OpenAPIRuntime/Errors/RuntimeError.swift

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 import protocol Foundation.LocalizedError
 import struct Foundation.Data
+import HTTPTypes
 
 /// Error thrown by generated code.
 internal enum RuntimeError: Error, CustomStringConvertible, LocalizedError, PrettyStringConvertible {
@@ -141,3 +142,25 @@ internal enum RuntimeError: Error, CustomStringConvertible, LocalizedError, Pret
 @_spi(Generated) public func throwUnexpectedResponseBody(expectedContent: String, body: any Sendable) throws -> Never {
     throw RuntimeError.unexpectedResponseBody(expectedContent: expectedContent, body: body)
 }
+
+/// HTTP Response status definition for ``RuntimeError``.
+extension RuntimeError: HTTPResponseConvertible {
+    /// HTTP Status code corresponding to each error case
+    public var httpStatus: HTTPTypes.HTTPResponse.Status {
+        switch self {
+        case .invalidServerURL, .invalidServerVariableValue, .pathUnset: .notFound
+        case .invalidExpectedContentType, .unexpectedContentTypeHeader: .unsupportedMediaType
+        case .missingCoderForCustomContentType: .unprocessableContent
+        case .unexpectedAcceptHeader: .notAcceptable
+        case .failedToDecodeStringConvertibleValue, .invalidAcceptSubstring, .invalidBase64String,
+            .invalidHeaderFieldName, .malformedAcceptHeader, .missingMultipartBoundaryContentTypeParameter,
+            .missingOrMalformedContentDispositionName, .missingRequiredHeaderField,
+            .missingRequiredMultipartFormDataContentType, .missingRequiredQueryParameter, .missingRequiredPathParameter,
+            .missingRequiredRequestBody, .unsupportedParameterStyle:
+            .badRequest
+        case .handlerFailed, .middlewareFailed, .missingRequiredResponseBody, .transportFailed,
+            .unexpectedResponseStatus, .unexpectedResponseBody:
+            .internalServerError
+        }
+    }
+}

Tests/OpenAPIRuntimeTests/Errors/Test_RuntimeError.swift

@@ -0,0 +1,77 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2024 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import HTTPTypes
+@_spi(Generated) @testable import OpenAPIRuntime
+import XCTest
+
+struct MockRuntimeErrorHandler: Sendable {
+    var failWithError: (any Error)? = nil
+    func greet(_ input: String) async throws -> String {
+        if let failWithError { throw failWithError }
+        guard input == "hello" else { throw TestError() }
+        return "bye"
+    }
+
+    static let requestBody: HTTPBody = HTTPBody("hello")
+    static let responseBody: HTTPBody = HTTPBody("bye")
+}
+
+final class Test_RuntimeError: XCTestCase {
+    func testRuntimeError_withUnderlyingErrorNotConforming_returns500() async throws {
+        let server = UniversalServer(
+            handler: MockRuntimeErrorHandler(failWithError: RuntimeError.transportFailed(TestError())),
+            middlewares: [ErrorHandlingMiddleware()]
+        )
+        let response = try await server.handle(
+            request: .init(soar_path: "/", method: .post),
+            requestBody: MockHandler.requestBody,
+            metadata: .init(),
+            forOperation: "op",
+            using: { MockRuntimeErrorHandler.greet($0) },
+            deserializer: { request, body, metadata in
+                let body = try XCTUnwrap(body)
+                return try await String(collecting: body, upTo: 10)
+            },
+            serializer: { output, _ in fatalError() }
+        )
+        XCTAssertEqual(response.0.status, .internalServerError)
+    }
+
+    func testRuntimeError_withUnderlyingErrorConforming_returnsCorrectStatusCode() async throws {
+        let server = UniversalServer(
+            handler: MockRuntimeErrorHandler(failWithError: TestErrorConvertible.testError("Test Error")),
+            middlewares: [ErrorHandlingMiddleware()]
+        )
+        let response = try await server.handle(
+            request: .init(soar_path: "/", method: .post),
+            requestBody: MockHandler.requestBody,
+            metadata: .init(),
+            forOperation: "op",
+            using: { MockRuntimeErrorHandler.greet($0) },
+            deserializer: { request, body, metadata in
+                let body = try XCTUnwrap(body)
+                return try await String(collecting: body, upTo: 10)
+            },
+            serializer: { output, _ in fatalError() }
+        )
+        XCTAssertEqual(response.0.status, .badGateway)
+    }
+}
+
+enum TestErrorConvertible: Error, HTTPResponseConvertible {
+    case testError(String)
+    /// HTTP status code for error cases
+    public var httpStatus: HTTPTypes.HTTPResponse.Status { .badGateway }
+}