fix: preserve priority fast mode

[neubig]

Summary

• recognize Codex's canonical priority service-tier response as Fast mode
• retain compatibility with the legacy fast response value
• type session metadata according to the app-server's string response
• cover both response forms in the Fast-mode session tests

Root cause

Current Codex normalizes both fast and priority configuration values to
priority before returning thread/start or thread/resume. codex-acp only
initialized fastModeEnabled when that response equaled fast, so the first
prompt sent serviceTier: null and cleared a configured Fast tier.

Impact

Fast mode configured through CODEX_CONFIG or config.toml now remains enabled
for new and resumed sessions. ACP clients that cannot directly set the
fast-mode session option, including OpenHands Agent Canvas, can rely on Codex
configuration instead.

Closes #285.

Validation

• npm test — 298 passed, 28 skipped
• npm run typecheck
• npm run build
• live Codex 0.144.1 app-server reproduction confirmed serviceTier: "priority"

Live evidence

Run on 2026-07-13 UTC on Linux x86_64 with Node v24.15.0 and the real @openai/codex 0.144.1 app-server. The reusable harness is committed as .pr/live_priority_fast_mode.ts; it starts the actual Codex subprocess, initializes the production CodexAcpClient and CodexAcpServer, creates an ACP session with the same {service_tier: "priority"} object supplied by CODEX_CONFIG, sends one real prompt, and records only service-tier/config state and the final response.

The identical command was used for both checkouts:

SOURCE_ROOT="$CHECKOUT" \
TESTED_COMMIT="$(git -C "$CHECKOUT" rev-parse HEAD)" \
node_modules/.bin/tsx .pr/live_priority_fast_mode.ts

Current main: canonical priority is cleared

Commit: 75893cd87741b2fe127f6698a8741a2e625c3787

Codex accepted the configured tier and returned canonical serviceTier: "priority" from thread/start, but codex-acp initialized its Fast option as off. The first real prompt then sent serviceTier: null, clearing the configured tier:

{
  "codexVersion": "0.144.1",
  "configuredServiceTier": "priority",
  "appServerServiceTier": "priority",
  "fastConfigOption": {"type": "select", "currentValue": "off"},
  "fastModeEnabledAfterStart": false,
  "firstPromptServiceTier": null,
  "promptStopReason": "end_turn",
  "finalMessage": "priority-fast-live-ok"
}

PR head: Fast mode remains enabled

Commit: dee5214f2bb53fcd7f8f8a383826de47228c56d7

The same Codex response now initializes Fast mode on, preserves enabled state, and sends the legacy-compatible Fast tier on the first live prompt:

{
  "codexVersion": "0.144.1",
  "configuredServiceTier": "priority",
  "appServerServiceTier": "priority",
  "fastConfigOption": {"type": "select", "currentValue": "on"},
  "fastModeEnabledAfterStart": true,
  "firstPromptServiceTier": "fast",
  "promptStopReason": "end_turn",
  "finalMessage": "priority-fast-live-ok"
}

Both runs issued the production initialize, thread/start, model/list, and turn/start JSON-RPC sequence. On the final PR head, npm run typecheck, npm test (298 passed, 28 skipped), and npm run build also passed.