fix: 孤代理三入口堵漏+精确断言+截断清洗换序

- _utf8_ok 助手：event_id/path/title 含孤代理（"\ud800"）在入口即拒
  （原会流到下游 encode 必 500）；props 字节守门 try/except 归入 properties_invalid
- 补零覆盖原因码测试：path=123 → invalid_field_type；utm 三层嵌套 → properties_depth
- 子串断言（"sensitive" in r 等）全改精确等值断言，码即 API
- path 改为先 sanitize_url 再截 500：中文查询参数 urlencode 膨胀最多 3 倍，
  先截后洗会破坏出门 ≤MAX_PATH_LEN 不变量（实测汉*200 查询洗后截回 500）
- docstring 写明就地写回契约（Task 6 ingest 依赖）

Co-Authored-By: Claude Code <noreply@anthropic.com>

server/tracker/tests/test_validation.py

@@ -17,8 +17,7 @@ def test_unknown_event_name():
     assert validate_event(_ev(event_name="xxx")) == "event_name_not_allowed"
 
 def test_sensitive_property_rejected():
-    r = validate_event(_ev(properties={"user_password": "1"}))
-    assert "sensitive" in r
+    assert validate_event(_ev(properties={"user_password": "1"})) == "sensitive_field"
 
 def test_shape_type_and_whitelist():
     assert validate_event(["x"]) == "event_invalid_shape"
@@ -26,14 +25,11 @@ def test_shape_type_and_whitelist():
     assert validate_event(_ev(properties={"foo": "bar"})) == "property_not_allowed"
 
 def test_too_many_props_and_long_string():
-    r = validate_event(_ev(properties={f"k{i}": "v" for i in range(40)}))
-    assert "properties" in r
-    r = validate_event(_ev(properties={"referrer": "x" * 600}))
-    assert "properties" in r
+    assert validate_event(_ev(properties={f"k{i}": "v" for i in range(40)})) == "properties_invalid"
+    assert validate_event(_ev(properties={"referrer": "x" * 600})) == "properties_too_long"
 
 def test_future_and_stale_time():
-    r = validate_event(_ev(occurred_at=NOW + timedelta(days=2)))
-    assert "occurred_at" in r                        # 超窗拒绝
+    assert validate_event(_ev(occurred_at=NOW + timedelta(days=2))) == "occurred_at_out_of_window"  # 超窗拒绝
     assert validate_event(_ev(occurred_at=NOW - timedelta(hours=2))) is None  # 窗内迟到可收
 
 def test_sanitize_url_drops_sensitive_params():
@@ -42,8 +38,7 @@ def test_sanitize_url_drops_sensitive_params():
     assert "monkey=1" in u          # fullmatch：含 key 子串的正常参数不误伤
 
 def test_nested_sensitive_key_rejected():
-    r = validate_event(_ev(properties={"utm": {"utm_a": 1, "user_password": "x"}}))
-    assert "sensitive" in r         # utm 在 page_view 白名单内，嵌套敏感键仍须命中
+    assert validate_event(_ev(properties={"utm": {"utm_a": 1, "user_password": "x"}})) == "sensitive_field"  # utm 在白名单内，嵌套敏感键仍须命中
 
 def test_naive_datetime_rejected():
     assert validate_event(_ev(occurred_at=NOW.replace(tzinfo=None))) == "invalid_occurred_at"
@@ -57,3 +52,19 @@ def test_nested_string_length_capped():
 
 def test_falsy_non_dict_properties_rejected():
     assert validate_event(_ev(properties=[])) == "properties_invalid"
+
+def test_lone_surrogate_event_id_rejected():
+    assert validate_event(_ev(event_id="\ud800")) == "invalid_event_id"
+
+def test_lone_surrogate_field_rejected():
+    assert validate_event(_ev(path="/p\ud800")) == "invalid_field_type"
+    assert validate_event(_ev(title="t\ud800")) == "invalid_field_type"
+
+def test_lone_surrogate_props_rejected():
+    assert validate_event(_ev(properties={"utm": {"a": "\ud800"}})) == "properties_invalid"
+
+def test_non_str_path_rejected():
+    assert validate_event(_ev(path=123)) == "invalid_field_type"
+
+def test_prop_depth_exceeded():
+    assert validate_event(_ev(properties={"utm": {"a": {"b": 1}}})) == "properties_depth"