fix: avoid re-using AuthInstanceID for sub agents (#22196)

Parent agents were re-using AuthInstanceID when spawning child agents.
This caused GetWorkspaceAgentByInstanceID to return the most recently
created sub agent instead of the parent when the parent tried to refetch
its own manifest.

Fix by not reusing AuthInstanceID for sub agents, and updating
GetWorkspaceAgentByInstanceID to filter them out entirely.

coderd/database/queries/workspaceagents.sql

@@ -17,6 +17,8 @@ WHERE
 	auth_instance_id = @auth_instance_id :: TEXT
 	-- Filter out deleted sub agents.
 	AND deleted = FALSE
+	-- Filter out sub agents, they do not authenticate with auth_instance_id.
+	AND parent_id IS NULL
 ORDER BY
 	created_at DESC;