### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When a LangGraph agent (or any agent using `custom_llm_provider`) is configured with **Method 3 convention-based header forwarding** (`x-a2a-{alias}-{target}`), the forwarded headers are silently dropped and never reach the downstream backend.

Admin-allowlisted headers (`extra_headers` on the agent config) are also affected.

### Steps to Reproduce

Configure a LangGraph agent in the proxy and send a request with:

x-a2a-my-agent-authorization: Bearer my-token The Authorization header will not be forwarded to the LangGraph backend despite the agent name matching.

### Relevant log output

```shell

```

### What part of LiteLLM is this about?

Proxy

### What LiteLLM version are you on ?

1.85.0

### Twitter / LinkedIn details

https://www.linkedin.com/in/maarmenta/