Fix Eio collector to work accross domains

[shonfeder]

The initial implementation of the Eio collector I introduced in #98 was not usable with signals emitted across domains. This was due to the fact that Eio.Switch.t is not thread-safe, and cannot be used across domains, but the implementation passed a switch into the Collector.Backend, which is then accessed across domains:

ocaml-opentelemetry/src/client-cohttp-eio/opentelemetry_client_cohttp_eio.ml

Lines 476 to 479 in 46d1289

	let create_backend ~sw ?(stop = Atomic.make false) ?(config = Config.make ())
	(env : Eio_unix.Stdenv.base) : (module OT.Collector.BACKEND) =
	let module E = (val mk_emitter ~sw ~stop ~config ~net:env#net ()) in
	(module Backend (E))

Consequently, the core fix introduced here is just to avoid sending a switch created from one domain into a reference that can be accessed from other domains.

However, it took me a bit of careful thinking thru the code flow to fully isolate this problem, and along the way I was able to yield a cleaned implementation.

I also made changes to the integration tests of two sorts:

1. Introduced a test cases that runs the Eio collector across multiple domains, to regressions.
2. Changed emit1_eio to produce deterministic signals even when run across multiple domains.

[shonfeder]

The test failure here is just a networking error: https://github.com/imandra-ai/ocaml-opentelemetry/actions/runs/16682099040/job/47222974012?pr=103#step:6:263

[shonfeder]

Gentle ping here, just to keep in contending in the queue :D

[c-cube]

Excellent, thank you. Sorry it took me a while to review it.