Add non-spawning create variants for TCP/TLS/serial clients

[jadamcrain]

Summary

Adds non-spawning (create_*) variants of the client channel constructors alongside the existing spawn_* functions, so callers can spawn the channel task on their own runtime and attach their own tracing instrumentation.

New public functions:

• create_tcp_client_task_with_options(host, retry, listener, client_options) -> (Channel, ClientTask)
• create_tls_client_task_with_options(host, retry, tls_config, listener, client_options) -> (Channel, ClientTask) (enable-tls)
• create_rtu_client_task(path, serial_settings, max_queued_requests, retry, decode, listener) -> (Channel, ClientTask) (serial)

Design

• ClientTask is a single opaque public type shared across all three transports — internally an enum over TcpChannelTask/SerialChannelTask — exposing only pub async fn run(self). No boxing.
• The default tracing spans were moved out of the create_*_channel helpers and into the spawn_* functions, so created tasks carry no span and the caller is free to wrap ClientTask::run with their own instrumentation, e.g. tokio::spawn(task.run().instrument(my_span)).
• Task construction is now eager (the task struct is built in create_* rather than inside an async move block). This is a pure struct build with no I/O; connection state transitions still happen in run().

Bug fix

The TLS connection path was labeling its tracing span "Modbus-Client-TCP"; it is now correctly "Modbus-Client-TLS".

Verification

cargo fmt, cargo check, and cargo clippy all pass clean with --features serial,enable-tls. Reviewed by Codex with no correctness findings.

[gemini-code-assist]

Code Review

This pull request introduces a new ClientTask type and corresponding create_* functions (create_tcp_client_task_with_options, create_rtu_client_task, and create_tls_client_task_with_options) to allow creating Modbus client tasks without spawning them immediately. This allows callers to apply custom instrumentation before spawning. The review feedback suggests optimizing spawn_tcp_channel and spawn_tls_channel to avoid cloning host (which can cause heap allocations) by creating the tracing span first and then moving host into the channel creation functions.