[Keda710]

Fixes #299.

Change

• Stop dispatching new shards after the first dispatch failure, while retaining receivers for shards already accepted.
• Drain every retained completion receiver, recording a disconnected channel as an error instead of returning early.
• Return the first error after draining, while preserving successful result ordering.
• Add mock-protocol tests for a later dispatch failure, a disconnected completion channel, and the normal success path.

Validation

• cargo test --offline --locked -p openlake_kv_client --lib client::tests:: (3 passed)
• cargo clippy --offline --locked --workspace --all-targets -- -D warnings (passed)
• cargo fmt --all -- --check (passed)
• git diff --check (passed)

Full cargo test --workspace on macOS hits an existing PyO3 extension-library link error (Undefined symbols for architecture arm64 for Python symbols). The Rust library tests above pass.