fix avatar-url to text (#26068)

api/migrations/versions/2025_09_17_1515-68519ad5cd18_knowledge_pipeline_migrate.py

     sa.Column('plugin_id', sa.String(length=255), nullable=False),
     sa.Column('auth_type', sa.String(length=255), nullable=False),
     sa.Column('encrypted_credentials', postgresql.JSONB(astext_type=sa.Text()), nullable=False),
-    sa.Column('avatar_url', sa.String(length=255), nullable=True),
+    sa.Column('avatar_url', sa.Text(), nullable=True),
     sa.Column('is_default', sa.Boolean(), server_default=sa.text('false'), nullable=False),
     sa.Column('expires_at', sa.Integer(), server_default='-1', nullable=False),
     sa.Column('created_at', sa.DateTime(), server_default=sa.text('CURRENT_TIMESTAMP'), nullable=False),

api/models/oauth.py

     plugin_id: Mapped[str] = db.Column(db.String(255), nullable=False)
     auth_type: Mapped[str] = db.Column(db.String(255), nullable=False)
     encrypted_credentials: Mapped[dict] = db.Column(JSONB, nullable=False)
-    avatar_url: Mapped[str] = db.Column(db.String(255), nullable=True, default="default")
+    avatar_url: Mapped[str] = db.Column(db.Text, nullable=True, default="default")
     is_default: Mapped[bool] = db.Column(db.Boolean, nullable=False, server_default=db.text("false"))
     expires_at: Mapped[int] = db.Column(db.Integer, nullable=False, server_default="-1")