Add assert-based pytest coverage for recent fixes

- tests/conftest.py: put repo root on sys.path so `import src...` resolves
  under pytest regardless of invocation directory.
- test_matrix_formatting.py: lock in HTML formatted_body + plain-text fallback,
  URL linkification, HTML escaping, and send_alert field rendering.
- test_station_persistence.py: cover default-load, save/reload round-trip
  (incl. Thai text), runtime-file precedence, and atomic-write cleanup.

These are real assert-based tests (unlike the existing print-style scripts) so
CI can gate on them. 13 tests, all passing.

tests/test_station_persistence.py

@@ -0,0 +1,77 @@
+"""Assert-based tests for station-mapping persistence.
+
+Station CRUD must survive restarts: the scraper loads its mapping from a
+runtime-writable JSON file (falling back to bundled defaults) and writes it back
+atomically. These tests exercise that load/save behaviour without constructing a
+full scraper (which would open network/DB connections).
+"""
+
+from src.water_scraper_v3 import EnhancedWaterMonitorScraper as Scraper
+
+
+def _bare_scraper(config_path):
+    """A scraper instance with only the station-config attribute set.
+
+    Bypasses __init__ so no database/HTTP connection is attempted.
+    """
+    scraper = Scraper.__new__(Scraper)
+    scraper.station_config_path = config_path
+    return scraper
+
+
+def test_loads_bundled_defaults_when_runtime_file_absent(tmp_path):
+    scraper = _bare_scraper(str(tmp_path / "does_not_exist.json"))
+    mapping = scraper._load_station_mapping()
+
+    assert len(mapping) == 16
+    assert mapping["8"]["code"] == "P.1"
+    assert mapping["8"]["english_name"] == "Nawarat Bridge"
+
+
+def test_save_then_reload_roundtrips_including_thai(tmp_path):
+    path = str(tmp_path / "stations.json")
+    scraper = _bare_scraper(path)
+    scraper.station_mapping = {
+        "1": {
+            "code": "P.99",
+            "thai_name": "สถานีทดสอบ",
+            "english_name": "Test Station",
+            "latitude": 1.0,
+            "longitude": 2.0,
+            "geohash": None,
+        }
+    }
+
+    assert scraper.save_stations() is True
+
+    reloaded = _bare_scraper(path)._load_station_mapping()
+    assert reloaded == scraper.station_mapping
+    assert reloaded["1"]["thai_name"] == "สถานีทดสอบ"
+
+
+def test_runtime_file_takes_precedence_over_defaults(tmp_path):
+    path = str(tmp_path / "stations.json")
+    writer = _bare_scraper(path)
+    writer.station_mapping = {"1": {"code": "ONLY"}}
+    assert writer.save_stations() is True
+
+    mapping = _bare_scraper(path)._load_station_mapping()
+    assert list(mapping.keys()) == ["1"]
+    assert mapping["1"]["code"] == "ONLY"
+
+
+def test_save_returns_false_without_a_path():
+    scraper = _bare_scraper("")
+    scraper.station_mapping = {}
+    assert scraper.save_stations() is False
+
+
+def test_save_is_atomic_no_tmp_left_behind(tmp_path):
+    path = tmp_path / "stations.json"
+    scraper = _bare_scraper(str(path))
+    scraper.station_mapping = {"1": {"code": "P.1"}}
+
+    assert scraper.save_stations() is True
+    assert path.exists()
+    # The temp file used during the atomic write must not remain.
+    assert not (tmp_path / "stations.json.tmp").exists()