feat: add upload_attachment MCP tool

Add support for uploading file attachments to Grist documents:

- GristClient.upload_attachment() method using multipart/form-data
- upload_attachment tool function with base64 decoding and MIME detection
- Tool registration in server.py
- Comprehensive unit tests (7 new tests)

Returns attachment ID for linking to records via update_records.

Bumps version to 1.3.0.

tests/unit/test_server.py

@@ -52,13 +52,14 @@ tokens:
     assert "add_column" in tool_names
     assert "modify_column" in tool_names
     assert "delete_column" in tool_names
+    assert "upload_attachment" in tool_names
 
     # Session tools (always registered)
     assert "get_proxy_documentation" in tool_names
     assert "request_session_token" in tool_names
 
-    # Should have all 14 tools
-    assert len(result.root.tools) == 14
+    # Should have all 15 tools
+    assert len(result.root.tools) == 15
 
 
 @pytest.mark.asyncio