Fix MockTransport not setting elapsed property

MockTransport now automatically sets response.elapsed to timedelta(0)
for both sync and async requests, matching the behavior of real
transports and eliminating the need for boilerplate code in handlers.

Previously, accessing response.elapsed on MockTransport responses
would raise RuntimeError. Now it returns timedelta(0) by default,
which is the expected behavior for mocked requests with no actual
network I/O.

Fixes #3712

httpx/_transports/mock.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import datetime
 import typing
 
 from .._models import Request, Response
@@ -24,6 +25,7 @@ class MockTransport(AsyncBaseTransport, BaseTransport):
         response = self.handler(request)
         if not isinstance(response, Response):  # pragma: no cover
             raise TypeError("Cannot use an async handler in a sync Client")
+        response.elapsed = datetime.timedelta()
         return response
 
     async def handle_async_request(
@@ -40,4 +42,5 @@ class MockTransport(AsyncBaseTransport, BaseTransport):
         if not isinstance(response, Response):
             response = await response
 
+        response.elapsed = datetime.timedelta()
         return response

tests/client/test_client.py

@@ -460,3 +460,40 @@ def test_client_decode_text_using_explicit_encoding():
         assert response.reason_phrase == "OK"
         assert response.encoding == "ISO-8859-1"
         assert response.text == text
+
+
+def test_mock_transport_elapsed():
+    """Test that MockTransport sets the elapsed property.
+
+    Regression test for https://github.com/encode/httpx/issues/3712
+    """
+    def handler(request):
+        return httpx.Response(200, json={"text": "Hello, world!"})
+
+    transport = httpx.MockTransport(handler)
+    with httpx.Client(transport=transport) as client:
+        response = client.get("http://www.example.com")
+        assert response.status_code == 200
+        # MockTransport should set elapsed to timedelta(0)
+        assert response.elapsed == timedelta(0)
+        # Accessing elapsed should not raise RuntimeError
+        assert isinstance(response.elapsed, timedelta)
+
+
+@pytest.mark.anyio
+async def test_async_mock_transport_elapsed():
+    """Test that MockTransport sets the elapsed property for async requests.
+
+    Regression test for https://github.com/encode/httpx/issues/3712
+    """
+    def handler(request):
+        return httpx.Response(200, json={"text": "Hello, world!"})
+
+    transport = httpx.MockTransport(handler)
+    async with httpx.AsyncClient(transport=transport) as client:
+        response = await client.get("http://www.example.com")
+        assert response.status_code == 200
+        # MockTransport should set elapsed to timedelta(0)
+        assert response.elapsed == timedelta(0)
+        # Accessing elapsed should not raise RuntimeError
+        assert isinstance(response.elapsed, timedelta)