feat: add managed agent license limit checks (#18937)

- Adds a query for counting managed agent workspace builds between two
timestamps
- The "Actual" field in the feature entitlement for managed agents is
now populated with the value read from the database
- The wsbuilder package now validates AI agent usage against the limit
when a license is installed

Closes coder/internal#777

coderd/database/queries/licenses.sql

@@ -35,3 +35,28 @@ DELETE
 FROM licenses
 WHERE id = $1
 RETURNING id;
+
+-- name: GetManagedAgentCount :one
+-- This isn't strictly a license query, but it's related to license enforcement.
+SELECT
+	COUNT(DISTINCT wb.id) AS count
+FROM
+	workspace_builds AS wb
+JOIN
+	provisioner_jobs AS pj
+ON
+	wb.job_id = pj.id
+WHERE
+	wb.transition = 'start'::workspace_transition
+	AND wb.has_ai_task = true
+	-- Only count jobs that are pending, running or succeeded. Other statuses
+	-- like cancel(ed|ing), failed or unknown are not considered as managed
+	-- agent usage. These workspace builds are typically unusable anyway.
+	AND pj.job_status IN (
+		'pending'::provisioner_job_status,
+		'running'::provisioner_job_status,
+		'succeeded'::provisioner_job_status
+	)
+	-- Jobs are counted at the time they are created, not when they are
+	-- completed, as pending jobs haven't completed yet.
+	AND wb.created_at BETWEEN @start_time::timestamptz AND @end_time::timestamptz;