chore(coderd/database): enforce agent name unique within workspace build (#18052)

Adds a database trigger that runs on insert and update of the
`workspace_agents` table. The trigger ensures that the agent name is
unique within the context of the workspace build it is being inserted
into.

coderd/database/migrations/000332_workspace_agent_name_unique_trigger.down.sql

@@ -0,0 +1,2 @@
+DROP TRIGGER IF EXISTS workspace_agent_name_unique_trigger ON workspace_agents;
+DROP FUNCTION IF EXISTS check_workspace_agent_name_unique();

coderd/database/migrations/000332_workspace_agent_name_unique_trigger.up.sql

@@ -0,0 +1,45 @@
+CREATE OR REPLACE FUNCTION check_workspace_agent_name_unique()
+RETURNS TRIGGER AS $$
+DECLARE
+	workspace_build_id uuid;
+	agents_with_name int;
+BEGIN
+	-- Find the workspace build the workspace agent is being inserted into.
+	SELECT workspace_builds.id INTO workspace_build_id
+	FROM workspace_resources
+	JOIN workspace_builds ON workspace_builds.job_id = workspace_resources.job_id
+	WHERE workspace_resources.id = NEW.resource_id;
+
+	-- If the agent doesn't have a workspace build, we'll allow the insert.
+	IF workspace_build_id IS NULL THEN
+		RETURN NEW;
+	END IF;
+
+	-- Count how many agents in this workspace build already have the given agent name.
+	SELECT COUNT(*) INTO agents_with_name
+	FROM workspace_agents
+	JOIN workspace_resources ON workspace_resources.id = workspace_agents.resource_id
+	JOIN workspace_builds ON workspace_builds.job_id = workspace_resources.job_id
+	WHERE workspace_builds.id = workspace_build_id
+		AND workspace_agents.name = NEW.name
+		AND workspace_agents.id != NEW.id;
+
+	-- If there's already an agent with this name, raise an error
+	IF agents_with_name > 0 THEN
+		RAISE EXCEPTION 'workspace agent name "%" already exists in this workspace build', NEW.name
+			USING ERRCODE = 'unique_violation';
+	END IF;
+
+	RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER workspace_agent_name_unique_trigger
+	BEFORE INSERT OR UPDATE OF name, resource_id ON workspace_agents
+	FOR EACH ROW
+	EXECUTE FUNCTION check_workspace_agent_name_unique();
+
+COMMENT ON TRIGGER workspace_agent_name_unique_trigger ON workspace_agents IS
+'Use a trigger instead of a unique constraint because existing data may violate
+the uniqueness requirement. A trigger allows us to enforce uniqueness going
+forward without requiring a migration to clean up historical data.';