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

docs/reference/api/builds.md

@@ -1767,6 +1767,17 @@ curl -X POST http://coder-server:8080/api/v2/workspaces/{workspace}/builds \
 {
   "dry_run": true,
   "log_level": "debug",
+  "on_success": {
+    "rich_parameter_values": [
+      {
+        "name": "string",
+        "value": "string"
+      }
+    ],
+    "template_version_id": "0ba39c92-1f1b-4c32-aa3e-9925d7713eb1",
+    "template_version_preset_id": "512a53a7-30da-446e-a1fc-713c630baff1",
+    "transition": "start"
+  },
   "orphan": true,
   "reason": "dashboard",
   "rich_parameter_values": [