Expose Codex service tiers for Langfuse pricing

[wallacelim]

Context

Codex service tiers can change model pricing, but the tracing plugin currently drops the active tier. As a result, Langfuse cannot distinguish Standard, Fast/Priority, Flex, or catalog-defined tiers when inferring cost.

Codex records tier changes in thread_settings_applied.service_tier. The first turn of a session may precede that event, so the plugin also needs a configuration fallback. Langfuse pricing-tier conditions operate on numeric usage details, while the raw tier remains useful as metadata.

What this changes

• captures the per-turn Codex service tier without creating an empty synthetic turn for settings events
• falls back to global/project Codex config.toml for an initial turn whose rollout omits the tier
• supports an explicit service_tier / LANGFUSE_CODEX_SERVICE_TIER fallback override
• records the raw tier as codex.service_tier generation metadata
• emits a generic numeric selector for non-default tiers, e.g. codex_service_tier_priority = 1
• documents how to use the selector in a Langfuse custom-model pricing condition

The plugin does not hard-code provider prices, credit conversions, or organization-specific discounts. Each Langfuse project remains responsible for its own model rate cards.

Example

A Fast-mode rollout that reports service_tier: "priority" emits:

{
  "metadata": {
    "codex.service_tier": "priority"
  },
  "usageDetails": {
    "codex_service_tier_priority": 1
  }
}

A custom model can select its higher-rate tier with a condition such as ^codex_service_tier_(fast|priority)$ > 0.

Scope and PR split

This PR is intentionally independent of:

• fix: normalize cached and reasoning usage details #28, which normalizes cached-input and reasoning-output usage buckets
• Fix Langfuse hook cwd fail for Windows #33, which fixes installed hook path resolution

Those correctness/reliability changes should remain separately reviewable. The generated bundle is included because this repository requires committed build output.

Validation

• corepack pnpm@9.5.0 test — 38 passing
• corepack pnpm@9.5.0 run lint — Prettier, TypeScript, and generated-bundle checks pass

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