Fix middleware error responses failing outputSchema validation

[jacksandom]

Summary

• Fix middleware error responses failing MCP SDK outputSchema validation
• Raise ToolError instead of returning ToolResult for error cases so the MCP SDK sets isError=True, bypassing outputSchema validation
• Update tests to assert ToolError is raised and fix incorrect CancelledError test

Context

PR #411 expanded the middleware to catch all exceptions and return them as ToolResult. Since ToolResult is a success-path object, the MCP SDK treats these as successful responses missing structuredContent and rejects them with:

Output validation error: outputSchema defined but no structured output returned

Before #411, general exceptions propagated to the MCP SDK which handled them correctly with isError=True. #411 attempted to set structured_content on error responses but reverted it because error dicts don't match tool-specific output schemas.

The fix raises ToolError (from fastmcp.exceptions) which propagates to the MCP SDK's exception handler → _make_error_result(str(e)) → CallToolResult(isError=True), bypassing validation entirely while preserving the structured JSON error payloads.

Test plan

• All 5 middleware unit tests pass
• Manually trigger a tool error (e.g. manage_jobs with action="bogus") and verify the actual error JSON is returned instead of the outputSchema validation error