fix: import ToolDefinitionManifest at runtime in ToolType model

Move the ToolDefinitionManifest import out of TYPE_CHECKING so SQLAlchemy
can resolve the string-annotated relationship when configuring the ToolType
mapper during startup.

Quality gates: py_compile and ruff passed.

apps/api/src/models/tool_type.py

@@ -6,8 +6,9 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from src.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
 
+from src.models.tool_definition_manifest import ToolDefinitionManifest
+
 if TYPE_CHECKING:
-    from src.models.tool_definition_manifest import ToolDefinitionManifest
     from src.models.user import User