fix: import ContentBlock under TYPE_CHECKING to avoid circular import

exceptions.py is imported during mcp package init (before mcp.types is
registered), so a top-level 'from mcp.types import ContentBlock' fails with
ModuleNotFoundError. content is only used as an annotation, so guard the
import with TYPE_CHECKING and add 'from __future__ import annotations'.

Author: Raed Lazreg

src/mcp/server/mcpserver/exceptions.py

@@ -1,6 +1,14 @@
 """Custom exceptions for MCPServer."""
 
-from mcp.types import ContentBlock
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    # Imported under TYPE_CHECKING to avoid a circular import: this module is
+    # pulled in during `mcp` package initialization (before `mcp.types` is
+    # registered), and `content` only needs `ContentBlock` as an annotation.
+    from mcp.types import ContentBlock
 
 
 class MCPServerError(Exception):