McpError does not support string messages as input

Describe the bug
McpError does not properly handle string messages passed during initialization.

To Reproduce
Steps to reproduce the behavior:
Calling raise McpError("Error message") results in an AttributeError, because the string object does not have a message attribute.

Python 3.11.9 (main, May 12 2025, 20:04:08) [GCC 13.3.0] on linux
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>>> from mcp.shared.exceptions import McpError
>>> raise McpError("Error")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".pyenv/versions/atheris-env/lib/python3.11/site-packages/mcp/shared/exceptions.py", line 13, in __init__
    super().__init__(error.message)
                     ^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'message'

There are multiple call sites in the MCP server codebase where McpError is raised with a plain string argument, leading to this error.

Expected behavior
According to the MCP SDK specification, McpError is expected to receive an ErrorData object, which includes a message attribute.

I believe either the SDK specification or its usage should be corrected:

Reproduce the failure with raise McpError("Error") and inspect mcp/shared/exceptions.py, along with the server call sites that pass plain strings. Resolve whether the intended input is a string or an ErrorData object, then add coverage for the chosen behavior and verify those call sites no longer raise an AttributeError.