[API/MCP] "User not found" should suggest the matching member id when a display name is passed

When an API or MCP call passes a user's display name instead of the user id (e.g. assignees: ["Rain"] while the id is Rain_Playernet), Quire returns 400: assignees: User not found: Rain with no hint. AI agents then retry the same name or give up, because the display name is all they see in task lists (assignees: ["Rain"] in compact search results).

Steps to reproduce

1. Call create_task / update_task / create_subtask with assignees: ["<display name of a project member>"], where the display name differs from the user id.
2. Or call get_user with that display name.

Expected: an error that helps fix the call, e.g. User not found: Rain. Did you mean Rain_Playernet? (matching project or organization members by display name). Actual: User not found: Rain (or 404: Not found. from get_user).

Notes

• In our AI agent sessions in October 2026 this caused 17 failed calls across create_subtask, bulk_create_subtasks, bulk_update_tasks, create_task, update_task and get_user.
• We are not asking to accept display names as input, since they are not unique; a hint in the error is enough.
• Compact task rows show assignee display names only, so returning ids there too (e.g. {id, name}) would also prevent the mistake.

Request: include matching member ids in the "User not found" error, and ideally return assignee ids in compact task rows.