fix(coderd): reject pinning child chats in patchChat handler

[johnstcn]

The UI already prevents child (delegated/subagent) chats from being pinned, but the PATCH /api/experimental/chats/{chat} endpoint did not enforce this. A direct API call could pin a child chat.

• Add a 400 Bad Request guard in patchChat when pinOrder > 0 and the chat has a ParentChatID
• Add TestChatPinOrder/RejectsChildChat test

Decision log

Decision	Rationale
400 (not 403 or 422)	Matches existing "Cannot pin an archived chat." pattern
Message: "Cannot pin a child chat."	Matches "Cannot pin an archived chat." style
Placement: after archived check	Groups related pin guards together
Test in TestChatPinOrder	Keeps all pin-order tests co-located
Backend-only	UI already prevents this

🤖

[johnstcn]

self-reviewed

[DanielleMaywood]

thoughts on enforcing at the schema-level to prevent a regression by a possible introduction of another pin-path?

[johnstcn]

thoughts on enforcing at the schema-level to prevent a regression by a possible introduction of another pin-path?

Yeah but we should roll other related items in as well e.g. the archival stuff. You could also make the argument that child chats belong in their own table.

[johnstcn]

49e2421 adds a check constraint on both pin order and archived