Add test for context provider API.

tests/test_context.py

@@ -2,6 +2,7 @@
 
 import typing as t
 
+import markupsafe
 import pytest
 
 from htpy import Context, Node, div
@@ -33,6 +34,19 @@ def test_context_provider(render: RenderFixture) -> None:
     assert render(result) == ["<div>", "Hello: c!", "</div>"]
 
 
+class Test_provider_outer_api:
+    """Ensure provider implements __iter__/__str__"""
+
+    def test_iter(self) -> None:
+        result = letter_ctx.provider("c", lambda: div[display_letter("Hello")])
+        assert list(result) == ["<div>", "Hello: c!", "</div>"]
+
+    def test_str(self) -> None:
+        result = str(letter_ctx.provider("c", lambda: div[display_letter("Hello")]))
+        assert result == "<div>Hello: c!</div>"
+        assert isinstance(result, markupsafe.Markup)
+
+
 def test_no_default(render: RenderFixture) -> None:
     with pytest.raises(
         LookupError,