Send single-message requests as a sized body

[nblintao]

Server-streaming calls, and unary calls over gRPC and gRPC-Web, send exactly one request message, but both clients hand it to the transport as a streamed body:

• On HTTP/1.1 the request goes out with Transfer-Encoding: chunked, although its size is known before the call starts.
• In the async client, pyqwest writes the headers and then pulls the body from an async generator, which runs on the event loop. If the loop is busy right after the call is made, the server has the headers but no body until the loop gets back to the generator. A proxy in front of the server that times out idle request bodies cuts such a request off.

This change encodes the message up front and passes the bytes to stream(), as unary Connect calls already do with post(), so the transport writes the whole request with a Content-Length. Client-streaming and bidi-streaming requests still stream. connect-go sends server-streaming requests the same way since connectrpc/connect-go#649, and connect-web sends them as a single body too.

Validation:

• New tests record the content each kind of call hands to the transport, for the async and sync clients under all three protocols: bytes for unary and server-streaming calls, an iterator for client and bidi streams. They fail without the change.
• On a real socket, with the event loop blocked for 2 s right after a server-streaming call starts: before this change the headers arrived with transfer-encoding: chunked and the body 2 s later; after it, headers and body arrive in the same read, with content-length.
• uv run poe check and uv run poe test-conformance pass.

🤖 Generated with Claude Code

[anuraaga]

Thanks @nblintao! I was worried this would be quite tricky but the approach looks pretty good. There is one point we have to remember for the future, if the public APIs accept list in the future for convenience reasons, I added a comment about it to not forget