Make the deeply nested body test independent of stack size

The test posted a well-formed 100,000-deep JSON array and asserted
PARSE_ERROR, which only holds where json.loads hits its recursion guard.
On CPython 3.14 with a large enough C stack the body parses, the server
correctly answers INVALID_REQUEST, and the assertion fails.

Leave the body unterminated so it is unparseable on every interpreter:
RecursionError where the guard trips, JSONDecodeError where the scanner
reaches the end of input. The server maps both to 400 + PARSE_ERROR.

Fixes #3146

Author: maxisbey

tests/server/test_streamable_http_modern.py

@@ -1139,8 +1139,9 @@ async def broken_list(ctx: ServerRequestContext, params: PaginatedRequestParams
 
 
 async def test_modern_post_with_deeply_nested_body_is_parse_error_not_a_crash() -> None:
-    """Deep nesting makes json.loads raise RecursionError; still an unparseable body: 400 + PARSE_ERROR."""
-    body = b"[" * 100_000 + b"]" * 100_000
+    """Unterminated deep nesting makes json.loads raise RecursionError or, where the stack is deep enough
+    to reach the end of input, JSONDecodeError; an unparseable body either way: 400 + PARSE_ERROR."""
+    body = b"[" * 100_000
     async with _asgi_client(_x_mcp_server()) as http:
         response = await http.post("/mcp", content=body, headers={"content-type": "application/json"})
     assert response.status_code == 400