Handle request stream ending before receiving request end (#54)

Motivation:

In #44, we introduced a `fatalError` in
`HTTPRequestConcludingAsyncReader` when the request stream ends before
the request end part is received. This is incorrect behaviour, and the
server should not fatal error in this scenario.

Modifications:

Replaced the `fatalError` with a `throw`.

Result:

The server no longer fatal errors when the request stream ends before
the request end part is received.

Author: aryan-25

Sources/NIOHTTPServer/HTTPRequestConcludingAsyncReader.swift

@@ -95,7 +95,7 @@ public struct HTTPRequestConcludingAsyncReader: ConcludingAsyncReader, ~Copyable
                             fatalError("Unexpectedly received a request head.")
 
                         case .none:
-                            fatalError("The stream unexpectedly ended before receiving a request end.")
+                            throw RequestBodyReadError.streamEndedBeforeReceivingRequestEnd
 
                         case .body(let element):
                             bodyElement = element

Sources/NIOHTTPServer/RequestBodyReadError.swift

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift HTTP Server open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift HTTP Server project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift HTTP Server project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+enum RequestBodyReadError: Error, CustomStringConvertible {
+    case streamEndedBeforeReceivingRequestEnd
+
+    var description: String {
+        switch self {
+        case .streamEndedBeforeReceivingRequestEnd:
+            "The request stream unexpectedly ended before receiving a request end part."
+        }
+    }
+}