Add unit tests for outer-document-only pk rename

Pure unit tests on `Mongo.Ecto.Conversions.from_ecto_pk/2` and
`inject_params/3` covering the recursion fix:

- top-level `:id` is renamed to `:_id`
- a nested map's `:id` is left alone even when the parent pk is `:id`
- the same holds inside a list of nested maps
- `pk: nil` is a no-op

Verified by reverting `conversions.ex` to the pre-patch version: 3 of
the 5 tests fail (nested `:id` ends up as `:_id`). With the patch they
all pass.

Author: scottmessinger

test/mongo_ecto/conversions_test.exs

@@ -0,0 +1,72 @@
+defmodule Mongo.Ecto.ConversionsTest do
+  use ExUnit.Case, async: true
+
+  alias Mongo.Ecto.Conversions
+
+  describe "from_ecto_pk/2" do
+    test "renames the primary key to :_id at the top level" do
+      assert {:ok, encoded} = Conversions.from_ecto_pk(%{id: "abc", title: "t"}, :id)
+
+      assert Map.new(encoded) == %{_id: "abc", title: "t"}
+    end
+
+    test "does not rename :id inside a nested map when the parent pk is :id" do
+      input = %{
+        id: "abc",
+        title: "outer",
+        jurisdiction: %{id: "MD", title: "Maryland"}
+      }
+
+      assert {:ok, encoded} = Conversions.from_ecto_pk(input, :id)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:_id] == "abc"
+      refute Map.has_key?(encoded_map, :id)
+
+      # The nested :id field must be passed through unchanged — it belongs to
+      # the embedded sub-document, not the parent schema.
+      assert Map.new(encoded_map[:jurisdiction]) == %{id: "MD", title: "Maryland"}
+    end
+
+    test "does not rename :id inside a list of nested maps" do
+      input = %{id: "abc", items: [%{id: "one"}, %{id: "two"}]}
+
+      assert {:ok, encoded} = Conversions.from_ecto_pk(input, :id)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:_id] == "abc"
+
+      assert Enum.map(encoded_map[:items], &Map.new/1) == [
+               %{id: "one"},
+               %{id: "two"}
+             ]
+    end
+
+    test "leaves nested maps alone when the pk is nil" do
+      input = %{id: "abc", jurisdiction: %{id: "MD"}}
+
+      assert {:ok, encoded} = Conversions.from_ecto_pk(input, nil)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:id] == "abc"
+      assert Map.new(encoded_map[:jurisdiction]) == %{id: "MD"}
+    end
+  end
+
+  describe "inject_params/3" do
+    test "renames the primary key to :_id only at the top level" do
+      input = %{
+        id: "abc",
+        jurisdiction: %{id: "MD", title: "Maryland"}
+      }
+
+      assert {:ok, encoded} = Conversions.inject_params(input, {}, :id)
+
+      encoded_map = Map.new(encoded)
+      assert encoded_map[:_id] == "abc"
+      refute Map.has_key?(encoded_map, :id)
+
+      assert Map.new(encoded_map[:jurisdiction]) == %{id: "MD", title: "Maryland"}
+    end
+  end
+end