feat: add workspace restart functionality to API (#25757)

This models restart as durable orchestration of existing stop and
start workspace builds instead of adding a new restart transition.
Keeping restart as two existing transitions preserves the current
build/provisioner model.

The child start build is created only after the parent stop build
succeeds, rather than being inserted immediately in a pending
state. That keeps `workspace_builds` aligned with actual
provisioner-ready work and avoids introducing a second
pending-build lifecycle that the provisioner and build acquisition
paths would need to understand.

Refs: https://linear.app/codercom/issue/PLAT-143

coderd/database/dbtestutil/workspacebuildorchestrations.go

@@ -0,0 +1,32 @@
+package dbtestutil
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+
+	"github.com/coder/coder/v2/coderd/database"
+)
+
+// GetWorkspaceBuildOrchestrationByParentBuildID reads a workspace
+// build orchestration row directly from the database for tests.
+//
+// It scans into the struct by column name so new columns are picked up
+// automatically without updating this helper.
+func GetWorkspaceBuildOrchestrationByParentBuildID(
+	ctx context.Context,
+	sqlDB *sql.DB,
+	parentBuildID uuid.UUID,
+) (database.WorkspaceBuildOrchestration, error) {
+	db := sqlx.NewDb(sqlDB, "postgres")
+	var orchestration database.WorkspaceBuildOrchestration
+	err := db.GetContext(
+		ctx,
+		&orchestration,
+		`SELECT * FROM workspace_build_orchestrations WHERE parent_build_id = $1`,
+		parentBuildID,
+	)
+	return orchestration, err
+}