Add a plain end-to-end test for the 2026-07-28 stateless tools/call

A pinned ClientSession over streamable_http_client against a real Server
mounted in-process, just the call and the result. The instrumented sibling
test pins the wire contract; this is the readable user-story version.

Author: maxisbey

tests/interaction/transports/test_hosting_http_modern.py

@@ -198,6 +198,25 @@ async def call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) ->
     assert "kaboom" not in error.message
 
 
+@requirement("hosting:http:modern:tools-call-stateless")
+async def test_pinned_client_calls_a_tool_on_a_modern_server() -> None:
+    """The simplest end-to-end exercise: SDK client to SDK server, ``tools/call``, 2026-07-28 stateless.
+
+    A pinned ``ClientSession`` over ``streamable_http_client`` against a real ``Server`` mounted
+    in-process; just the call and the result. The wire contract (headers, envelope, no handshake,
+    no session id) is pinned by the instrumented sibling test below.
+    """
+    with anyio.fail_after(5):
+        async with (
+            mounted_app(_server()) as (http, _),
+            streamable_http_client(f"{BASE_URL}/mcp", http_client=http, protocol_version=MODERN_VERSION) as (r, w),
+            ClientSession(r, w, protocol_version=MODERN_VERSION) as session,
+        ):
+            result = await session.call_tool("add", {"a": 2, "b": 3})
+
+    assert result == snapshot(CallToolResult(content=[TextContent(text="5")], result_type="complete"))
+
+
 @requirement("hosting:http:modern:tools-call-stateless")
 @requirement("lifecycle:stateless:request-envelope")
 @requirement("lifecycle:stateless:caller-meta-preserved")