fix: use Code.ensure_compiled/1 for compile-time module checks

[Munksgaard]

Code.ensure_loaded?/1 only checks if a module is already loaded in the BEAM VM. When called at compile time (e.g. in module body expressions or via use Jido.Composer.Workflow), it returns false for modules that exist in the project but have not been compiled yet, causing spurious "not a valid Jido.Action module" errors.

For example, the following triggers the bug because MapNode.new/1 runs at compile time and ExtractDocumentAction may not have been compiled yet:

defmodule MyApp.ExtractionPipeline do
  {:ok, extract_node} =
    Jido.Composer.Node.MapNode.new(
      name: :extract,
      over: :documents,
      node: MyApp.ExtractDocumentAction
    )

use Jido.Composer.Workflow,
    name: "extraction_pipeline",
    nodes: %{extract: extract_node, merge: MyApp.MergeAction},
    transitions: %{{:extract, :ok} => :merge, {:merge, :ok} => :done},
    initial: :extract
end

Replace with Code.ensure_compiled/1 which triggers compilation of the target module if needed. Only changes call sites that can run at compile time; ensure_loaded? calls for optional runtime deps (AgentServer, OpenTelemetry) are left unchanged.

[Munksgaard]

Hm, this doesn't seem to work the way I expected. Sorry for the noise.