fix(service): Return 400 for client-side upload stream errors

[jan-auer]

When a client drops the connection mid-upload, the broken stream error was propagating as Error::Io and returning 500. The server is not at fault here — this should be a 400.

Dedicated stream type

Introduces ClientError and ClientStream as a distinct type from the outgoing PayloadStream. Previously, both incoming and outgoing streams were BoxStream<'static, io::Result<Bytes>>, making it impossible to distinguish a client-side failure from a backend I/O error at the type level.

ClientStream is now the type for all incoming request bodies. ClientError wraps the underlying axum/hyper body error and is Clone (backed by Arc).

Distinct status code

Adds Error::Client(ClientError) to the service error enum. The server maps this variant to 400 Bad Request, while all other I/O errors remain 500.

Unpacking client errors

Backends receive a ClientStream, convert it into a reqwest::Body or io::Reader for storage, and need to detect when a failure was caused by the client rather than the backend.

common::unpack_client_error walks the error source chain and checks two locations: the error itself (direct ClientError), and custom io::Error payloads (where StreamReader or reqwest/hyper may have wrapped it). All backends now call this after a failed put and return Error::Client when the source was the upload stream.

Generics

Several stream utilities were generalised over the stream and error type:

• SizedPeek — previously hardcoded to PayloadStream; now avoids intermediate boxing
• MeteredPayloadStream and Services::meter_stream — now a single generic implementation that works for both stream types and avoids an extra inner Box