topic_mentions: Fix restriction rule for @-topic mentions.

Now, the topic wildcard mention follows the following
rules:
* If the topic has less than 15 participants , anyone
can use @ topic mentions.
* For more than 15, the org setting 'wildcard_mention_policy'
determines who can use @ topic mentions.

Earlier, topic wildcard mentions followed the same restriction
as stream wildcard mentions, which was incorrect.

Fixes part of #27700.

zerver/lib/exceptions.py

@@ -47,6 +47,8 @@ class ErrorCode(Enum):
     REACTION_DOES_NOT_EXIST = auto()
     SERVER_NOT_READY = auto()
     MISSING_REMOTE_REALM = auto()
+    TOPIC_WILDCARD_MENTION_NOT_ALLOWED = auto()
+    STREAM_WILDCARD_MENTION_NOT_ALLOWED = auto()
 
 
 class JsonableError(Exception):
@@ -584,3 +586,27 @@ class MissingRemoteRealmError(JsonableError):  # nocoverage
     @override
     def msg_format() -> str:
         return _("Organization not registered")
+
+
+class StreamWildcardMentionNotAllowedError(JsonableError):
+    code: ErrorCode = ErrorCode.STREAM_WILDCARD_MENTION_NOT_ALLOWED
+
+    def __init__(self) -> None:
+        pass
+
+    @staticmethod
+    @override
+    def msg_format() -> str:
+        return _("You do not have permission to use stream wildcard mentions in this stream.")
+
+
+class TopicWildcardMentionNotAllowedError(JsonableError):
+    code: ErrorCode = ErrorCode.TOPIC_WILDCARD_MENTION_NOT_ALLOWED
+
+    def __init__(self) -> None:
+        pass
+
+    @staticmethod
+    @override
+    def msg_format() -> str:
+        return _("You do not have permission to use topic wildcard mentions in this topic.")