fix: add default "hard" to enforcement in SetBudgetRequest OpenAPI spec

CodeRabbit requested that the enforcement property include a default value
to document that the handler defaults it to "hard" when omitted. This makes
the spec self-documenting and consistent with the handler behavior in api.go.

https://claude.ai/code/session_01EifZWMXeBB35K4is7bFZqj

Author: claude

packages/engine/internal/server/docs/swagger.json

@@ -854,6 +854,7 @@
             "type": "object",
             "properties": {
                 "enforcement": {
+                    "default": "hard",
                     "description": "\"hard\" or \"warn\"",
                     "type": "string"
                 },

packages/engine/internal/server/docs/swagger.yaml

@@ -131,6 +131,7 @@ definitions:
   server.SetBudgetRequest:
     properties:
       enforcement:
+        default: "hard"
         description: '"hard" or "warn"'
         type: string
       monthly_token_limit: