Fast mode is disabled when Codex reports the canonical priority service tier

[neubig]

Problem

Codex normalizes both the legacy fast service-tier value and the canonical
priority value to priority in its effective configuration. Consequently,
thread/start and thread/resume return serviceTier: "priority" for a Fast
session.

codex-acp 1.1.2 initializes fastModeEnabled with
sessionMetadata.currentServiceTier === "fast". A session configured through
CODEX_CONFIG={"service_tier":"fast"},
CODEX_CONFIG={"service_tier":"priority"}, or the equivalent config.toml
therefore starts with Fast mode marked off. On the first prompt, codex-acp sends
serviceTier: null, explicitly clearing the configured tier.

I reproduced this against Codex CLI 0.144.1: a thread/start request with
config.service_tier = "fast" returned serviceTier: "priority".

Expected behavior

Treat both fast (legacy ACP/app-server value) and priority (canonical
Codex/OpenAI value) as Fast mode enabled when initializing or resuming an ACP
session.

Impact

ACP clients that cannot call the fast-mode session config option directly
cannot reliably enable Fast mode through Codex configuration. This includes
OpenHands Agent Canvas, where CODEX_CONFIG otherwise provides a natural
configuration route.