Fix invalidate_search_cache using a key that never matches stored entries (#852)

invalidate_search_cache(query) built its cache key as
generate_cache_key(f"{query}|10|None"), but the write path
(searxng_search_results) replaces the caller's default count of 10 with the
admin-configured _get_result_count() (default 5) before building the key.

So a default search for "X" is cached under "X|5|None", while invalidation
looked for "X|10|None" — they never match, and invalidate_search_cache
silently failed to remove anything in the default configuration, violating
its docstring ("invalidate ... just the given query").

Derive the count from _get_result_count() so invalidation matches the
default-search entry the write path actually stores. The same bug (and fix)
applies to both the src/search and services/search copies.

Note: time-filtered variants (e.g. "X|5|day") still aren't reachable from a
query-only signature, since cache keys are opaque SHA-256 hashes with no
stored query; clearing those would need a broader cache-index redesign and is
out of scope here.

Adds tests/test_search_cache_invalidation.py covering the default-count case.

tests/test_search_cache_invalidation.py

@@ -0,0 +1,45 @@
+"""Regression test for invalidate_search_cache key construction.
+
+The write path (`searxng_search_results`) stores a cache entry under
+``generate_cache_key(f"{query}|{count}|{time_filter}")`` where ``count`` is the
+admin-configured result count (``_get_result_count()``, default **5**) — it
+replaces the caller's default of 10 with the configured value before building
+the key.
+
+The original ``invalidate_search_cache`` hardcoded ``f"{query}|10|None"``, so it
+never matched the key the write path actually produced (``|5|None`` by default)
+and silently failed to invalidate anything — a contract violation of its own
+docstring ("invalidate ... just the given query"). The fix derives the count
+from ``_get_result_count()`` so invalidation matches the stored default entry.
+"""
+import pytest
+
+from src.search import core
+from src.search.cache import generate_cache_key
+
+
+def test_invalidate_uses_configured_count_not_hardcoded_10(tmp_path, monkeypatch):
+    query = "python tutorial"
+    result_count = 5  # documented default of _get_result_count()
+
+    # Pin the configured count and redirect the cache dir to keep the test hermetic.
+    monkeypatch.setattr(core, "_get_result_count", lambda: result_count)
+    monkeypatch.setattr(core, "SEARCH_CACHE_DIR", tmp_path)
+
+    # Reproduce exactly what searxng_search_results writes for a default search:
+    # the caller's default count of 10 is replaced by result_count, time_filter=None.
+    write_key = generate_cache_key(f"{query}|{result_count}|None")
+    cache_file = tmp_path / f"{write_key}.cache"
+    cache_file.write_text("{}", encoding="utf-8")
+    core.search_cache_index[write_key] = None
+
+    try:
+        core.invalidate_search_cache(query)
+
+        assert not cache_file.exists(), (
+            "invalidate_search_cache failed to remove the entry the write path "
+            "stored under the configured result count — it used a mismatched key."
+        )
+        assert write_key not in core.search_cache_index
+    finally:
+        core.search_cache_index.pop(write_key, None)