fix: preserve anthropic auth header name

[mxyhi]

Summary

This fixes Anthropic-native /v1/messages/count_tokens requests when the inbound client authenticates with Authorization instead of x-api-key.

Problem

A request came in on /v1/messages/count_tokens through the Anthropic wire format and reached an Anthropic-compatible relay that requires Authorization: Bearer .... Token Proxy resolved the upstream as provider=anthropic, but the proxy layer always rewrote Anthropic upstream auth to x-api-key, which caused the relay to reject the forwarded request with Required request header 'Authorization' ... is not present.

Root Cause

resolve_request_auth only retained Anthropic credential values, not the original inbound header name. Later, resolve_upstream_auth unconditionally emitted x-api-key for every Anthropic provider, even when the inbound Anthropic-native request had arrived with Authorization and the target upstream expected that same header name.

Fix

The proxy now records the original auth header shape for Anthropic-native inbound paths (Authorization, x-api-key, or x-anthropic-api-key). When forwarding to an Anthropic provider, Token Proxy preserves that header name and only replaces the value with the configured upstream key when one exists. Non-Anthropic-native inbound paths keep the old fallback behavior so OpenAI/Gemini conversion paths are not changed accidentally.

Validation

I ran:

• cargo test -p token_proxy_core
• cargo test -p token_proxy_core proxy::http::tests -- --nocapture
• cargo test -p token_proxy_core anthropic_count_tokens_preserves_authorization_header_name_for_upstream -- --nocapture
• cargo test -p token_proxy_core anthropic_upstream_auth_reuses_x_api_key_header_name_with_upstream_key -- --nocapture
• cargo fmt --all --check