fix(coderd): mark sub agent deletion via boolean instead of delete (#18411)

Deletion of data is uncommon in our database, so the introduction of sub agents
and the deletion of them introduced issues with foreign key assumptions, as can
be seen in coder/internal#685. We could have only addressed the specific case by
allowing cascade deletion of stats as well as handling in the stats collector,
but it's unclear how many more such edge-cases we could run into.

In this change, we mark the rows as deleted via boolean instead, and filter them
out in all relevant queries.

Fixes coder/internal#685

coderd/database/queries/workspaces.sql

@@ -303,6 +303,8 @@ WHERE
 				WHERE
 					workspace_resources.job_id = latest_build.provisioner_job_id AND
 					latest_build.transition = 'start'::workspace_transition AND
+					-- Filter out deleted sub agents.
+					workspace_agents.deleted = FALSE AND
 					@has_agent = (
 						CASE
 							WHEN workspace_agents.first_connected_at IS NULL THEN
@@ -846,7 +848,11 @@ LEFT JOIN LATERAL (
 		workspace_agents.name as agent_name,
 		job_id
 	FROM workspace_resources
-	JOIN workspace_agents ON workspace_agents.resource_id = workspace_resources.id
+	JOIN workspace_agents ON (
+		workspace_agents.resource_id = workspace_resources.id
+		-- Filter out deleted sub agents.
+		AND workspace_agents.deleted = FALSE
+	)
 	WHERE job_id = latest_build.job_id
 ) resources ON true
 WHERE