uv + nats-server package changes

[wallyqs]

Follow up from a couple of the previous PRs:

• Move nats-server package from import nats.server into import nats.testing.server instead

[Jarema]

@wallyqs why we want to move it back to nats package?
It's a dev dependency for us, and very rarely for users.
In many clients we try to split those. And as javascript did recently - even split core nats from js, kv and object store.

[caspervonb]

Heya, so this seems to undo the addition of workspaces?

Keeping nats-server as its own package is beneficial for both us and the ecosystem, it's its own little standalone thing that exists without knowledge of anything but the existence of the nats-server binary being in path and some of its options.

Avoids cyclical dependencies for us as-well, easy to depend on siblings, hard to depend on a manifest package.

[wallyqs]

Updated to keep the workspace and instead only changed the import to nats.testing.server.
I think we need to add a matrix step on CI where we run tests with these deps: aiohttp, aiofiles, nkeys (websockets + nkeys tests)

[caspervonb]

• Made nats-server private, for the time being (possibly indefinitely).
• Got a draft adding the full matrix, but can't get mail parser to actually run Add uv_extras to nats package test matrix #750