fix rustdoc warnings on transport links

- Re-export TransportHandle / TransportReader / TransportWriter alongside
  SubprocessTransport. Without these in the public API, rustdoc refused
  to resolve [`TransportHandle::close`] from SubprocessTransport's docs.
- Inline the close()/end_input() link prose where the broken-link
  warning fired.

cargo doc --no-deps now builds clean.

src/lib.rs

@@ -125,7 +125,7 @@ pub use messages::{
 };
 pub use options::{ClaudeAgentOptions, Effort, McpServersConfig, PermissionMode, SystemPrompt};
 pub use query::query;
-pub use transport::SubprocessTransport;
+pub use transport::{SubprocessTransport, TransportHandle, TransportReader, TransportWriter};
 
 /// Crate version, as set in `Cargo.toml`. Sent to the CLI as
 /// `CLAUDE_AGENT_SDK_VERSION` in the subprocess env.