fix(typing): accept a built JsonValue in init_connection_params

[Danipulok]

What

Client(init_connection_params=...) is annotated Optional[dict], but a built JsonValue
works and is arguably the more direct thing to pass. session.py:195 converts only when the
value is a dict:

if isinstance(init_connection_params, dict):
    init_connection_params = utils.obj_to_jsonvalue(init_connection_params)

Anything else goes straight into InvokeWithLayer(..., params=init_connection_params). So a
caller who has already built a raw.types.JsonObject — because they wanted control over the
value types, or because they are forwarding one they received — passes it, it works, and
every type checker and IDE marks the call as wrong.

The change

The annotation, and the docstring line above it:

init_connection_params: Optional[Union[dict, "raw.base.JsonValue"]] = None,

No behaviour moves: the isinstance branch already draws exactly this distinction, and this
only writes down the half of it that was missing.