Word-boundary match for snippet and subject-term ranking (#1473 follow-up) (#2556)

#1473 converted the title and sports-hint matches in services/search/ranking.py
to word boundaries but left two raw substring tests:

  - snippet_score: 'term in snippet.lower()' — query term 'port' hits
    'transport'/'support', inflating a result's relevance.
  - news_quality_adjustment: 't in text or t in netloc' for the subject term —
    query 'us' substring-matches 'business'/'music', so an off-topic page
    wrongly escapes the off-topic penalty on a country/subject news query.

Add a _has_word helper (the same \b...\b pattern title_score already used) and
route all three word checks (title, snippet, subject) through it, so the file
stays consistent and a future partial fix can't reintroduce the same bug class.
Pure ranking refinement: scores change only for spurious substring matches; no
API or schema change.

(cherry picked from commit 22bd23f044f191bb30e43f6b68386552817f4cc3)

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

tests/test_search_ranking_subject_substring.py

@@ -0,0 +1,87 @@
+"""Regression: snippet and subject-term matching must be word-boundary.
+
+#1473 converted the title and sports-hint matches in ranking.py to word
+boundaries, but left two raw substring tests behind:
+
+  - snippet_score: ``term in snippet.lower()`` — query term "port" hits
+    "transport"/"support", inflating a result's relevance.
+  - news_quality_adjustment: ``t in text or t in netloc`` for the subject term —
+    query "us" substring-matches "business"/"music", so an off-topic page
+    wrongly escapes the off-topic penalty for a country/subject news query.
+
+Both now go through ``_has_word`` (the same ``\\b...\\b`` pattern title_score
+uses), so a short term no longer matches inside an unrelated word.
+
+``rank_search_results`` is exercised on both the services module (the
+/api/search path) and the src re-export shim (the agent web_search path).
+"""
+import pytest
+
+import services.search.ranking as services_ranking
+import src.search.ranking as src_ranking
+
+RANK_MODULES = [services_ranking, src_ranking]
+RANK_IDS = ["services", "src"]
+
+
+# --- _has_word helper (defined in the services module) ---------------------
+
+def test_has_word_rejects_substring_false_positives():
+    assert services_ranking._has_word("business and music", "us") is False
+    assert services_ranking._has_word("transport and support", "port") is False
+    assert services_ranking._has_word("passport office", "sport") is False
+
+
+def test_has_word_matches_standalone_terms():
+    assert services_ranking._has_word("the us economy", "us") is True
+    assert services_ranking._has_word("port forwarding guide", "port") is True
+
+
+# --- snippet_score: substring term must not inflate relevance ---------------
+
+@pytest.mark.parametrize("ranking", RANK_MODULES, ids=RANK_IDS)
+def test_snippet_substring_does_not_outrank_a_true_nonmatch(ranking):
+    # Non-news query so only snippet relevance differs (no news adjustment).
+    query = "port forwarding"
+    results = [
+        # C first: a genuine non-match (no query word at all).
+        {"title": "Networking notes", "snippet": "weather updates today",
+         "url": "https://example.org/c", "age": "1 day"},
+        # B: contains "port" only inside "transport"/"support" (substring).
+        {"title": "Networking notes", "snippet": "transport and support",
+         "url": "https://example.org/b", "age": "1 day"},
+    ]
+    ranked = ranking.rank_search_results(query, results)
+    # Pre-fix B got a spurious term hit and outranked C; post-fix they have the
+    # same (zero) snippet term match, so input order stands and C stays first.
+    assert ranked[0]["url"] == "https://example.org/c"
+
+
+# --- subject-term off-topic penalty: substring must not suppress it ---------
+
+@pytest.mark.parametrize("ranking", RANK_MODULES, ids=RANK_IDS)
+def test_offtopic_subject_substring_is_still_penalized(ranking):
+    # News query with subject term "us". B mentions "us" only inside
+    # "business"; A mentions "us" as a standalone word. The snippets are padded
+    # past the 200-char length cap and are otherwise identical, so both sides
+    # have equal base scores and the ONLY thing that can differ is the off-topic
+    # penalty — isolating the bug from incidental length/term scoring.
+    filler = (
+        "regional market report covered many provincial topics and figures in "
+        "detail over the period with extra commentary and analysis written for "
+        "readers wanting more depth on the matter at hand and well into the "
+        "following week ahead"
+    )
+    query = "us news"
+    results = [
+        # B first: off-topic, "us" only as a substring of "business".
+        {"title": "Daily roundup", "snippet": "business economy and policy. " + filler,
+         "url": "https://example.org/b", "age": "1 day"},
+        # A: on-topic, standalone "us".
+        {"title": "Daily roundup", "snippet": "us economy and policy. " + filler,
+         "url": "https://example.org/a", "age": "1 day"},
+    ]
+    ranked = ranking.rank_search_results(query, results)
+    # Pre-fix B escaped the off-topic penalty (substring "us") so the tie kept
+    # input order (B on top); post-fix B takes the -1.0 penalty and A rises.
+    assert ranked[0]["url"] == "https://example.org/a"