fix(knowledge): drop COLLATE utf8mb4_unicode_ci from knowledge-square join

DM8 rejects ``COLLATE utf8mb4_unicode_ci`` (-2007 Syntax error)
because the collation name does not exist outside MySQL. The clause
was patching an "Illegal mix of collations" error on MySQL, but
collation belongs to the schema definition (table charset/collate),
not the query. Business code should not have to specify it inline.

Drop the COLLATE wrapper entirely; keep the CAST since INT
``knowledge.id`` still has to be compared against VARCHAR
``space_channel_member.business_id``. MySQL deployments need the
table-level collate matched at table-creation time, which they
already do via ``mysql_collate='utf8mb4_unicode_ci'`` table args
on the joined models.

src/backend/bisheng/knowledge/domain/models/knowledge.py

@@ -3,7 +3,7 @@ from enum import Enum
 from typing import Any, List, Optional, Tuple, Union, Dict
 
 from pydantic import BaseModel, field_validator
-from sqlalchemy import Boolean, Integer, String, collate
+from sqlalchemy import Boolean, Integer, String
 from sqlmodel import Column, DateTime, Field, case, delete, func, or_, select, text, update
 from sqlmodel.sql.expression import Select, SelectOfScalar, col
 
@@ -767,6 +767,8 @@ class KnowledgeDao(KnowledgeBase):
 
         rejection_cutoff = datetime.now() - REJECTED_STATUS_DISPLAY_WINDOW
 
+        kid_str = col(Knowledge.id).cast(String)
+
         # Subquery: count subscribers (status=ACTIVE) per space
         subscriber_subq = (
             select(
@@ -791,13 +793,13 @@ class KnowledgeDao(KnowledgeBase):
             )
             .outerjoin(
                 SpaceChannelMember,
-                (collate(col(Knowledge.id).cast(String), 'utf8mb4_unicode_ci') == SpaceChannelMember.business_id)
+                (kid_str == SpaceChannelMember.business_id)
                 & (SpaceChannelMember.business_type == BusinessTypeEnum.SPACE)
                 & (SpaceChannelMember.user_id == user_id),
             )
             .outerjoin(
                 subscriber_subq,
-                collate(col(Knowledge.id).cast(String), 'utf8mb4_unicode_ci') == subscriber_subq.c.business_id,
+                kid_str == subscriber_subq.c.business_id,
             )
             .where(
                 Knowledge.type == KnowledgeTypeEnum.SPACE.value,