Add a flag to specify streaming requirements on the request body

Sources/HTTPAPIs/Client/HTTPClientRequestBody+Data.swift

@@ -20,14 +20,16 @@ public import struct Foundation.Data
 
 @available(macOS 26.2, iOS 26.2, watchOS 26.2, tvOS 26.2, visionOS 26.2, *)
 extension HTTPClientRequestBody where Writer: ~Copyable {
-    /// Creates a seekable request body from `Data`.
+    /// Creates a seekable request body from `Data` which does not require streaming.
     ///
     /// - Parameter data: The data to send as the request body.
     public static func data(_ data: Data) -> Self {
-        .seekable(knownLength: Int64(data.count)) { offset, writer in
+        var body = HTTPClientRequestBody.seekable(knownLength: Int64(data.count)) { offset, writer in
             var writer = writer
             try await writer.write(data.span.extracting(droppingFirst: Int(offset)))
             return nil
         }
+        body.requiresStreaming = false
+        return body
     }
 }

Sources/HTTPAPIs/Client/HTTPClientRequestBody.swift

@@ -68,6 +68,13 @@ where Writer.WriteElement == UInt8, Writer: SendableMetatype {
     /// the `Content-Length` header field.
     public let knownLength: Int64?
 
+    /// Whether the body has to be streamed, or can be collected into a buffer before sending.
+    ///
+    /// An HTTPClient backend implementation that does not support request body streaming will
+    /// throw if the body requires streaming. Both `restartable` and `seekable` bodies require
+    /// streaming by default.
+    public var requiresStreaming: Bool = true
+
     private enum WriteBody {
         case restartable(@Sendable (consuming Writer) async throws -> HTTPFields?)
         case seekable(@Sendable (Int64, consuming Writer) async throws -> HTTPFields?)