send_message
Send a message to WeCom group/chat via webhook.
This function sends a message to WeCom using the configured webhook URL.
The message will be formatted as markdown and added to message history.
Args:
content: The message content to send. Cannot be empty or whitespace.
ctx: Optional FastMCP context for logging and progress tracking.
Returns:
str: Success message if the message was sent successfully.
Raises:
ValueError: If content is empty/whitespace, webhook URL is not set,
or if there's an error sending the message.