[Service Bus] Support the Service Bus emulator

## Summary

The crate cannot talk to the Service Bus emulator. Tests and samples need a live Azure namespace.

## Motivation

The emulator lets a contributor run the tests with no Azure subscription, and it lets a caller develop offline. It listens without TLS on a local endpoint and it uses a fixed shared access key, so it needs both the connection string credential from #4956 and a way to disable TLS.

This lowers the barrier for outside contributors, which matters for a crate with as much remaining work as this one.

## Proposal

- Support a custom endpoint and a plain text connection to a local emulator.
- Document how to run the tests against the emulator.
- Note in the documentation which features the emulator does not implement.

## Dependencies

Blocked by:

- #4956 [Service Bus] Add connection string and shared access signature credentials

Sub-issue of #4934.

Start with the blocked dependency, #4956, for the connection string and shared access signature credential support, then review the Service Bus crate’s existing endpoint and TLS handling. Done means connecting to the emulator with a custom plain-text endpoint, documenting how to run tests against it, and listing unsupported emulator features.

Written by the indexing model from the issue text.