Fix: return type contract violation and doc typo in try_download_result tool

[hyun-hyang]

Bug Report (Issue substitute)

This PR documents and fixes a contract violation bug in the MCP tool
try_download_result.

Issues

1. Return tuple order and length mismatch with declared return type
2. Docstring typo ("bu" → "by")

This repository currently has Issues disabled, so this PR serves as
a bug report and fix combined.

Bug Evidence

1. Return type contract violation

Before
Declared return type:

rodin-api-mcp/src/rodin_mcp/rodin.py

Line 98 in 855b2a8

) -> str | tuple[str, Optional[MCPImage], str]:

Actual return value:

• rodin-api-mcp/src/rodin_mcp/rodin.py

  Line 136 in 855b2a8

  return "Task not finished yet. Try again later."

• rodin-api-mcp/src/rodin_mcp/rodin.py

  Lines 166 to 170 in 855b2a8

  	return (
  	returning_image,
  	"The image above is an preview image of the generated model, without texture and material.\n"
  	f"The assets are downloaded to {parameters.target_directory_path}"
  	)

  
  This violates both tuple length and positional ordering, which is critical
  in MCP environments where tuple order is semantically significant.

After (this PR)

- ) -> str | tuple[str, Optional[MCPImage], str]:
+ ) -> str | tuple[Optional[MCPImage], str]:

2. Docstring typo

rodin-api-mcp/src/rodin_mcp/rodin.py

Line 103 in 855b2a8

- Always make sure an absolute path is given bu the user before calling this tool.

Fix applied in this PR

- Always make sure an absolute path is given bu the user before calling this tool.
+ Always make sure an absolute path is given by the user before calling this tool.

Visual reference (GitHub UI diff)