add japanese phrase preprocessing

Code adapted from GSOC code by @miku.

test/python/api/query_processing/test_split_japanese_phrases.py

@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2025 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Tests for japanese phrase splitting.
+"""
+from pathlib import Path
+
+import pytest
+
+from icu import Transliterator
+
+import nominatim_api.search.query as qmod
+from nominatim_api.query_preprocessing.config import QueryConfig
+from nominatim_api.query_preprocessing import split_japanese_phrases
+
+def run_preprocessor_on(query):
+    proc = split_japanese_phrases.create(QueryConfig().set_normalizer(None))
+
+    return proc(query)
+
+
+@pytest.mark.parametrize('inp,outp', [('大阪府大阪市大阪', '大阪府:大阪市:大阪'),
+                                      ('大阪府大阪', '大阪府:大阪'),
+                                      ('大阪市大阪', '大阪市:大阪')])
+def test_split_phrases(inp, outp):
+    query = [qmod.Phrase(qmod.PhraseType.NONE, inp)]
+
+    out = run_preprocessor_on(query)
+
+    assert out == [qmod.Phrase(qmod.PhraseType.NONE, outp)]