gateway: price the Metis pass-through route from its reported tokens

The /v1/systemone pass-through is an EndpointType.GENERIC route: LiteLLM
logs it, but with no model on the route the only cost it can apply is the
flat cost_per_request (a budget estimate, not an invoice). So decisions
were billed at a fixed 0.0001 instead of the real $0.12 per 1M input
tokens GreenPT reports back in usage.input_tokens.

Price the route from its own response body via a new build-time patch,
patch_passthrough_token_cost.py, in the house style of the existing
patch_*.py files: any route listed in
general_settings.coop_passthrough_cost_per_token (USD per input token,
keyed by path) is billed as usage.input_tokens x rate. cost_per_request
stays as the fallback for a listed route whose response carries no token
count, and every unlisted route is unchanged. Auth is untouched.

config.yaml: list /v1/systemone at 0.00000012 ($0.12 per 1M input
tokens); update the metering note.

README.md

@@ -167,6 +167,7 @@ expect a 15–20 min maintenance window and **never panic-restart mid-migration*
 - Check the [LiteLLM changelog](https://github.com/BerriAI/litellm/releases) for breaking changes
 - The `start.sh` and `config.yaml` may need adjustment if LiteLLM changes its env-var or config schema
 - **Build-time patches ride on LiteLLM internals — verify each one after an upgrade.** `patch_budget_message.py`, `patch_image_cost.py` (keeps image-gen pricing on the shared cost-map key) and `patch_audio_request_cost.py` (adds the declared per-request charge on the audio paths: Tinfoil bills `voxtral-tts` $0.002 and `whisper-large-v3-turbo` $0.01 per request, which the cost map cannot express). Each prints `[coop-patch]` in the build log. They are non-fatal by design: a missing anchor warns and the build still succeeds, so the behaviour can vanish silently on a version bump. Related: code/litellm#5.
+- **`patch_passthrough_token_cost.py`** prices a generic pass-through route from the token count it reports in its own response body. A `pass_through_endpoints` target whose host matches none of vertex/anthropic/openai/gemini resolves to `EndpointType.GENERIC`: it is logged, but with no model on the route LiteLLM only knows the flat `cost_per_request`, which is a budget estimate, not an invoice. The patch bills any route listed in `general_settings.coop_passthrough_cost_per_token` (USD per input token, keyed by path) as `usage.input_tokens × rate` — Metis (`/v1/systemone`) at $0.12 per 1M input tokens. Same rules: non-fatal, `[coop-patch]` in the build log, verify after every upgrade. `cost_per_request` stays as the fallback for a listed route whose response carries no token count. The block prints `[coop-passthrough-cost] input_tokens=… rate=… -> response_cost=…` on each priced call, so the log is the field check that it fired.
 - The CORS env var is `LITELLM_CORS_ORIGINS` (upstream-native since 1.84.0); the old `LITELLM_CORS_ALLOWED_ORIGINS` + sed patch are dead — don't re-add them
 - Test on a staging instance if the version jump is large