fix: pony-alpha model status and SSE stream heartbeat issue

- Change openrouter/pony-alpha model status from 'alpha' to 'beta' to prevent deletion
- Fix ReferenceError where heartbeat was used before initialization in cleanupStream
- Declare heartbeat and streamTimeout with let before cleanupStream function
- Change const assignments to let assignments for timer variables

opencode/packages/opencode/test/tool/fixtures/models-api.json

@@ -27325,7 +27325,7 @@
         "open_weights": false,
         "cost": { "input": 0, "output": 0 },
         "limit": { "context": 200000, "output": 131000 },
-        "status": "alpha"
+        "status": "beta"
       },
       "z-ai/glm-4.7": {
         "id": "z-ai/glm-4.7",