fix(acp): tolerate missing token count for handled Codex commands #4079

Summary

An ACP conversation can fail when codex-acp handles an unknown slash command such as /codereview-roasted.

Reproduction

1. Start an ACPAgent conversation backed by @agentclientprotocol/codex-acp.
2. Send an unsupported slash command, for example /codereview-roasted this PR.
3. The adapter handles the command locally and returns a successful session/prompt response without a model turn or usage update.

Actual behavior

codex-acp returns usage: null and _meta.quota.token_count: null. ACPAgent._extract_token_usage() treats the null token count as a mapping and raises:

'NoneType' object has no attribute 'get'

The generic ACP turn-error path then emits ACPPromptError and marks the entire conversation ERROR, so it cannot continue normally.

Expected behavior

A successful handled command with no usage information should record zero token usage and complete the turn. Unknown commands should show the adapter-provided response without poisoning the conversation.

Proposed fix

Normalize nullable quota and token_count values to empty mappings before reading counters, with a regression test for {"quota": null} / {"quota": {"token_count": null}}.