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.sql.go

@@ -4286,6 +4286,44 @@ func (q *sqlQuerier) GetLicenses(ctx context.Context) ([]License, error) {
 	return items, nil
 }
 
+const getManagedAgentCount = `-- name: GetManagedAgentCount :one
+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 $1::timestamptz AND $2::timestamptz
+`
+
+type GetManagedAgentCountParams struct {
+	StartTime time.Time `db:"start_time" json:"start_time"`
+	EndTime   time.Time `db:"end_time" json:"end_time"`
+}
+
+// This isn't strictly a license query, but it's related to license enforcement.
+func (q *sqlQuerier) GetManagedAgentCount(ctx context.Context, arg GetManagedAgentCountParams) (int64, error) {
+	row := q.db.QueryRowContext(ctx, getManagedAgentCount, arg.StartTime, arg.EndTime)
+	var count int64
+	err := row.Scan(&count)
+	return count, err
+}
+
 const getUnexpiredLicenses = `-- name: GetUnexpiredLicenses :many
 SELECT id, uploaded_at, jwt, exp, uuid
 FROM licenses