MCP get_inbox fails when inbox tasks contain subtasks

Structured MCP get_inbox fails with MCP error -32602: Output validation error: Invalid structured content for tool get_inbox.

Repeated validation failure:

expected: string

code: invalid_type

path: tasks -> [task index] -> subtasks -> [subtask index] -> completed_at

message: Invalid input: expected string, received undefined

In the reproduced run, this affected inbox task indexes 7 through 13 across their incomplete subtasks. The incomplete subtasks omit completed_at, while the MCP output validator expects a string, so the entire get_inbox response is rejected.

Environment: Structured 4.5.3 on macOS 27.0.0. Diagnostic data has been sent separately to support@structured.app.