Do not accept pattern_type from user input in push rules. (#15088)

Internally the push rules module uses a `pattern_type` property for `event_match`
conditions (and `related_event_match`) to mark the condition as matching the
current user's Matrix ID or localpart.

This is leaky to the Client-Server API where a user can successfully set a condition
which provides `pattern_type` instead of `pattern` (note that there's no benefit to
doing this -- the user can just use their own Matrix ID or localpart instead). When
serializing back to the client the `pattern_type` property is converted into a proper
`pattern`.

The following changes are made to avoid this:

* Separate the `KnownCondition::EventMatch` enum value into `EventMatch`
  and `EventMatchType`, each with their own expected properties. (Note that a
  similar change is made for `RelatedEventMatch`.)
* Make it such that the `pattern_type` variants serialize to the same condition kind,
  but cannot be deserialized (since they're only provided by base rules).
* As a final tweak, convert `user_id` vs. `user_localpart` values into an enum.

tests/push/test_push_rule_evaluator.py

@@ -401,6 +401,33 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
             "pattern should not match before a newline",
         )
 
+    def test_event_match_pattern(self) -> None:
+        """Check that event_match conditions do not use a "pattern_type" from user data."""
+
+        # The pattern_type should not be deserialized into anything valid.
+        condition = {
+            "kind": "event_match",
+            "key": "content.value",
+            "pattern_type": "user_id",
+        }
+        self._assert_not_matches(
+            condition,
+            {"value": "@user:test"},
+            "should not be possible to pass a pattern_type in",
+        )
+
+        # This is an internal-only condition which shouldn't get deserialized.
+        condition = {
+            "kind": "event_match_type",
+            "key": "content.value",
+            "pattern_type": "user_id",
+        }
+        self._assert_not_matches(
+            condition,
+            {"value": "@user:test"},
+            "should not be possible to pass a pattern_type in",
+        )
+
     def test_exact_event_match_string(self) -> None:
         """Check that exact_event_match conditions work as expected for strings."""