feat: tag chat-originating agent logs with chat_id (#25019)

Workspace-agent logs emitted while serving chatd-driven requests were
not correlated with the originating chat, making agent logs hard to
attribute to the corresponding/originating chat.

This adds agent-side chat context middleware that parses `Coder-Chat-Id`
once, enriches agent access logs and structured handler/background logs,
and adds a chatd bridge log when chat headers are attached to an agent
connection.

Closes CODAGT-324

coderd/x/chatd/chatd.go

@@ -905,6 +905,12 @@ func (c *turnWorkspaceContext) getWorkspaceConn(ctx context.Context) (workspaces
 			})
 
 			c.mu.Unlock()
+			c.server.logger.Debug(ctx, "set chat headers on agent conn",
+				slog.F("chat_id", chatSnapshot.ID),
+				slog.F("ancestor_chat_ids", ancestorIDs),
+				slog.F("workspace_id", chatSnapshot.WorkspaceID.UUID),
+				slog.F("agent_id", dialResult.AgentID),
+			)
 			return agentConn, nil
 		}
 		currentConn = c.conn