[app-server] add thread/tokenUsage/updated v2 event

[celia-oai]

the TokenEvent event message becomes thread/tokenUsage/updated in v2.
before & after:

< {
<   "method": "codex/event/token_count",
<   "params": {
<     "conversationId": "019ab891-4c55-7790-9670-6c3b48c33281",
<     "id": "1",
<     "msg": {
<       "info": {
<         "last_token_usage": {
<           "cached_input_tokens": 3072,
<           "input_tokens": 5152,
<           "output_tokens": 16,
<           "reasoning_output_tokens": 0,
<           "total_tokens": 5168
<         },
<         "model_context_window": 258400,
<         "total_token_usage": {
<           "cached_input_tokens": 3072,
<           "input_tokens": 5152,
<           "output_tokens": 16,
<           "reasoning_output_tokens": 0,
<           "total_tokens": 5168
<         }
<       },
<       "rate_limits": {
<         "credits": null,
<         "primary": null,
<         "secondary": null
<       },
<       "type": "token_count"
<     }
<   }
< }
< {
<   "method": "thread/tokenUsage/updated",
<   "params": {
<     "threadId": "019ab891-4c55-7790-9670-6c3b48c33281",
<     "tokenUsage": {
<       "last": {
<         "cachedInputTokens": 3072,
<         "inputTokens": 5152,
<         "outputTokens": 16,
<         "reasoningOutputTokens": 0,
<         "totalTokens": 5168
<       },
<       "modelContextWindow": 258400,
<       "total": {
<         "cachedInputTokens": 3072,
<         "inputTokens": 5152,
<         "outputTokens": 16,
<         "reasoningOutputTokens": 0,
<         "totalTokens": 5168
<       }
<     },
<     "turnId": "1"
<   }
< }

[owenlin0]

small comments but lgtm!