add rerank top_n unit test

here is the result :

./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 16 items

unit/test_rerank.py::test_rerank PASSED                                                                                                             [  6%]
unit/test_rerank.py::test_rerank_tei_format PASSED                                                                                                  [ 12%]
unit/test_rerank.py::test_invalid_rerank_req[documents0] PASSED                                                                                     [ 18%]
unit/test_rerank.py::test_invalid_rerank_req[None] PASSED                                                                                           [ 25%]
unit/test_rerank.py::test_invalid_rerank_req[123] PASSED                                                                                            [ 31%]
unit/test_rerank.py::test_invalid_rerank_req[documents3] PASSED                                                                                     [ 37%]
unit/test_rerank.py::test_rerank_usage[Machine learning is-A machine-Learning is-19] PASSED                                                         [ 43%]
unit/test_rerank.py::test_rerank_usage[Which city?-Machine learning is -Paris, capitale de la-26] PASSED                                            [ 50%]
unit/test_rerank.py::test_rerank_top_n[None-4] PASSED                                                                                               [ 56%]
unit/test_rerank.py::test_rerank_top_n[2-2] PASSED                                                                                                  [ 62%]
unit/test_rerank.py::test_rerank_top_n[4-4] PASSED                                                                                                  [ 68%]
unit/test_rerank.py::test_rerank_top_n[99-4] PASSED                                                                                                 [ 75%]
unit/test_rerank.py::test_rerank_tei_top_n[None-4] PASSED                                                                                           [ 81%]
unit/test_rerank.py::test_rerank_tei_top_n[2-2] PASSED                                                                                              [ 87%]
unit/test_rerank.py::test_rerank_tei_top_n[4-4] PASSED                                                                                              [ 93%]
unit/test_rerank.py::test_rerank_tei_top_n[99-4] PASSED                                                                                             [100%]

=================================================================== 16 passed in 8.84s ===================================================================

Author: YannFollet

tools/server/tests/unit/test_rerank.py

@@ -102,3 +102,45 @@ def test_rerank_usage(query, doc1, doc2, n_tokens):
     assert res.status_code == 200
     assert res.body['usage']['prompt_tokens'] == res.body['usage']['total_tokens']
     assert res.body['usage']['prompt_tokens'] == n_tokens
+
+
+@pytest.mark.parametrize("top_n,expected_len", [
+    (None, len(TEST_DOCUMENTS)),  # no top_n parameter
+    (2, 2),
+    (4, 4),
+    (99, len(TEST_DOCUMENTS)),    # higher than available docs
+])
+def test_rerank_top_n(top_n, expected_len):
+    global server
+    server.start()
+    data = {
+        "query": "Machine learning is",
+        "documents": TEST_DOCUMENTS,
+    }
+    if top_n is not None:
+        data["top_n"] = top_n
+
+    res = server.make_request("POST", "/rerank", data=data)
+    assert res.status_code == 200
+    assert len(res.body["results"]) == expected_len
+
+
+@pytest.mark.parametrize("top_n,expected_len", [
+    (None, len(TEST_DOCUMENTS)),  # no top_n parameter
+    (2, 2),
+    (4, 4),
+    (99, len(TEST_DOCUMENTS)),    # higher than available docs
+])
+def test_rerank_tei_top_n(top_n, expected_len):
+    global server
+    server.start()
+    data = {
+        "query": "Machine learning is",
+        "texts": TEST_DOCUMENTS,
+    }
+    if top_n is not None:
+        data["top_n"] = top_n
+
+    res = server.make_request("POST", "/rerank", data=data)
+    assert res.status_code == 200
+    assert len(res.body) == expected_len