[Service Bus] Sender builds a new session and link for each call

## Summary

`Sender::schedule_message` creates an `AmqpSession`, begins it, attaches a sender link, sends, detaches, and ends the session, all inside one call. Other send paths repeat the pattern.

## Motivation

An attach and detach round trip per message adds latency to every send and gives the service no chance to reuse link credit. It also defeats the recoverable layer in #4940, because a link that only lives for one call can never be recovered.

Every attach uses the literal link name `sender-link`, so concurrent senders on one connection are not distinguishable in a service side trace.

## Proposal

- Attach the sender link once and hold it for the life of the `Sender`.
- Route sends, scheduled sends, and cancellations over the held link and the held management links.
- Give each link a unique name that includes the client identifier.

## Dependencies

Blocked by:

- #4940 [Service Bus] Add a recoverable connection and link layer

Sub-issue of #4934.

Start with Sender::schedule_message and inspect the other send paths that create, attach, detach, and end sessions. Review the recoverable connection and link layer from #4940 before proceeding. Done means Sender holds reusable sender and management links, all send operations use them, and link names include the client identifier.

Written by the indexing model from the issue text.