Fix #348: Allow ToolError to carry custom content and is_error

[Stoobyy]

Why

Closes #348. There is currently no way to set isError=True when returning non-text content from tool calls (e.g., images, mixed content). The only way to get isError=True is to let an unhandled exception propagate, which always produces a single TextContent block.

What changed

• ToolError now accepts content and is_error keyword arguments
• Tool.run() re-raises ToolError instead of wrapping it
• _handle_call_tool catches ToolError before the generic exception handler, constructing a CallToolResult with the caller's custom content and is_error value

Usage

from mcp.server.mcpserver.exceptions import ToolError
from mcp.types import TextContent, ImageContent

@server.tool()
def my_tool():
    raise ToolError(
        "Processing failed",
        content=[
            TextContent(type="text", text="Detailed error message"),
            ImageContent(type="image", data="base64...", mimeType="image/png"),
        ],
    )

Validation

• All 91 existing tests pass
• 2 new tests added: ToolError with custom content, ToolError with default content