[bug] first combo target can dispatch after request send-budget reservation is denied #5688

Client or integration

Direct HTTP/API client

Area

Proxy and routing

Summary

On official v2.63.0, the first combo target can still dispatch after the shared request-send budget refuses its reservation. Expected: denied first-target reservation makes zero upstream inference requests and returns the existing typed local 429 request_send_budget_exhausted. For later targets, denial should preserve the last real upstream failure without dispatching the denied target.

Reproduction

1. Configure a failover combo.
2. Arrange for the shared request send budget to be exhausted before the first combo dispatch.
3. Enter executeComboResponses.
4. Observe that a denied first reservation can fall through into physical dispatch.
5. Local regression coverage asserts first-target refusal => upstream hit count 0 and typed 429; later-target refusal => backup hit count 0 and prior upstream response preserved.

Version

2.63.0 / commit 96b1406

Operating system

Ubuntu Server Linux x86_64

Provider and model

Not provider-specific

Logs or error output

Expected local error code: request_send_budget_exhausted

Screenshots and supporting files

Related to PR #4609, which established the shared request-send budget. This report is the remaining combo first-target admission gap.

Redacted configuration

{"combo":{"strategy":"failover","targets":["provider-a/model-a","provider-b/model-b"]}}

Checks

• I searched existing issues and documentation.
• I removed secrets, tokens, account details, request credentials, and personal data.