client: add BidiStream::into_split for independently owned halves

[iainmcgin]

Summary

Adds BidiStream::into_split, splitting a bidirectional stream into independently owned halves so the two sides can be driven from separate tasks — true full duplex:

let (mut send, mut recv) = client.running_sum().await?.into_split();
let reader = tokio::spawn(async move {
    while let Some(reply) = recv.message().await? { /* ... */ }
    Ok::<_, connectrpc::ConnectError>(())
});
for req in requests {
    send.send(req).await?;
}
send.close_send();
reader.await.expect("reader task")?;

Builds on #227, now merged, and rebased onto main; additive — no existing API changes, no codegen changes (generated bidi methods still return BidiStream).

Design

BidiStream was already two disjoint sides internally; the split is a plain destructure with no locking added:

• BidiSendHalf<Req> owns send/close_send (and only needs Req bounds — looser than the combined impl). It carries its own copy of the whole-call deadline. Dropping it ends the request body cleanly; the RPC continues until the receive half finishes.
• BidiRecvHalf<B, RespView> owns message/headers/trailers/error and the lazy response-initialization state machine. The abort-on-drop of in-flight initialization tasks (client: make BidiStream receive initialization cancellation-safe #221) moves here from BidiStream's Drop — which also frees BidiStream of its Drop impl so into_split can destructure. Dropping this half cancels the RPC, exactly as dropping a whole BidiStream did; a code comment records that the send-before-recv field order is load-bearing for the drop sequence.
• BidiStream keeps its full API via thin delegation, and into_split sits in an unbounded impl (a pure move needs no bounds). No reunite — documented on both halves.
• Docs state the HTTP/2 requirement for interleaved use prominently on into_split itself (an HTTP/1.1 caller doing response-dependent sends would deadlock), and steer users toward spawned tasks over naming the halves' body type parameter.

Testing

• Three new e2e tests over a real gRPC/h2 connection: full-duplex ping-pong with the halves owned by different tasks (each echo received before the next send), dropping the send half ends the stream cleanly, dropping the receive half fails subsequent sends instead of hanging.
• All 562 all-features unit tests pass unchanged through the delegating methods, including the client: make BidiStream receive initialization cancellation-safe #221 cancellation-safety tests.
• Live socket drive: split ping-pong over h2 completes in ~50 ms alongside the existing client-stream probes.
• Conformance client suites re-run: Connect 2580/2580; gRPC shows only the known environment-flaky Timeouts cases (including unary cases this diff does not touch).
• cargo semver-checks vs 0.8.1: 196/196 (additive). Net non-test size: +177 lines.

[iainmcgin]

[claude code] Restacked onto the current #227 tip (f948aba), which picked up that PR review round. Rebase was clean — no conflicts with the doc and re-export changes there, despite both touching client/mod.rs. Branch is now 8950ea0; task lint and the full workspace suite (56 suites, 0 failures) are green on the restacked branch.

This is now targeted at the 0.9.0 release alongside #227, so it needs to land right after its parent.