simplify top_n init since no need to return error

result to test :

./tests.sh unit/test_rerank.py -v -x
==================================================== test session starts =====================================================
platform linux -- Python 3.12.3, pytest-8.3.5, pluggy-1.6.0 -- /home/yann/dev/yann/llama.cpp/tools/server/tests/test/bin/python3
cachedir: .pytest_cache
rootdir: /home/yann/dev/yann/llama.cpp/tools/server/tests
configfile: pytest.ini
plugins: anyio-4.11.0
collected 8 items

unit/test_rerank.py::test_rerank PASSED                                                                                [ 12%]
unit/test_rerank.py::test_rerank_tei_format PASSED                                                                     [ 25%]
unit/test_rerank.py::test_invalid_rerank_req[documents0] PASSED                                                        [ 37%]
unit/test_rerank.py::test_invalid_rerank_req[None] PASSED                                                              [ 50%]
unit/test_rerank.py::test_invalid_rerank_req[123] PASSED                                                               [ 62%]
unit/test_rerank.py::test_invalid_rerank_req[documents3] PASSED                                                        [ 75%]
unit/test_rerank.py::test_rerank_usage[Machine learning is-A machine-Learning is-19] PASSED                            [ 87%]
unit/test_rerank.py::test_rerank_usage[Which city?-Machine learning is -Paris, capitale de la-26] PASSED               [100%]

===================================================== 8 passed in 4.31s ======================================================

Author: YannFollet

tools/server/server.cpp

@@ -5084,10 +5084,7 @@ int main(int argc, char ** argv) {
             return;
         }
 
-        int top_n = documents.size(); // no top_n will return all the documents
-        if (body.count("top_n") == 1) {
-            top_n = body.at("top_n");
-        }
+        int top_n = json_value(body, "top_n", (int)documents.size());
 
         // create and queue the task
         json responses = json::array();