merlin: show behavior with non-default context

[jchavarri]

The test shows how dune ocaml merlin errors out when trying to get artifacts generated in a non-default context. Not sure if this is the expected behavior, but it might be helpful to have it documented.

[rgrinberg]

Yes, this is expected. There's a merlin field that can be used to determine which context is used for merlin.

[jchavarri]

I see, thanks for the pointers. I could find the test/blackbox-tests/test-cases/merlin/default-based-context.t test, which contains similar checks than the ones added in this PR.

For additional context 😅 I am investigating how to make dune choose one context or another more dynamically, as we would like to have ocaml-lsp query dune for artifacts in a specific context (as part of the work in #10222). For this, I understand something like the Dune context configuration would not be enough because it can't be changed from the clients. What I'm not sure about is if the change would need to be added in the interface between ocaml-lsp and dune (I understand, the ocaml-merlin server).

I could find the commands exposed by the ocaml-merlin server:

dune/bin/ocaml/ocaml_merlin.ml

Lines 27 to 29 in 6c40cb1

	| File of string
	| Halt
	| Unknown of string

Would it be necessary to add some parameter to the File command to specify the context merlin is querying for?