feat: remove chat chain mode (#26980)

Removes OpenAI Responses "chain mode" from chatd. Closes CODAGT-445.

- Deletes `chatopenai/responses.go` (chain detection, activation, prompt filtering, response ID extraction) and its tests.
- Deletes the `ChainBroken` classification in `chaterror` and the chatloop retry bookkeeping that disabled chain mode mid-generation.
- Drops the `chain_broken` label from the `coderd_chatd_stream_retries_total` metric.
- Stops reading and writing `chat_messages.provider_response_id`
- Deletes the dead `ClearChatMessageProviderResponseIDsByChatID` query. Dropping the column is a follow-up migration.
- Deletes three chatloop hooks no caller sets (`ReloadMessages`, `DisableChainMode`, `PrepareMessages`), the dead `const AgentChatContextSentinelPath`, and stale chain-mode comments.

🤖 Generated by Coder Agents on behalf of @johnstcn.

coderd/database/querier.go

@@ -88,7 +88,6 @@ type sqlcQuerier interface {
 	CleanTailnetLostPeers(ctx context.Context) error
 	CleanTailnetTunnels(ctx context.Context) error
 	CleanupDeletedMCPServerIDsFromChats(ctx context.Context) error
-	ClearChatMessageProviderResponseIDsByChatID(ctx context.Context, chatID uuid.UUID) error
 	CountAIBridgeSessions(ctx context.Context, arg CountAIBridgeSessionsParams) (int64, error)
 	CountAuditLogs(ctx context.Context, arg CountAuditLogsParams) (int64, error)
 	// Cheap queue-length check used by ChatMachine.Update when deciding