feat(ai-proxy): configurable credential attachment via auth field

[ndreno]

Motivation

ai-proxy's Provider conflated two orthogonal concerns: the wire protocol (OpenAI Chat Completions/Responses vs Anthropic Messages) and the auth header convention. The credential header was hardcoded in three separate places (OpenAI transport, Anthropic transport, and the /v1/models aggregator), so any OpenAI-compatible upstream with a non-standard credential header could not be used without minting a new provider variant.

Concrete trigger: Brave AI Grounding exposes an OpenAI-compatible /v1/chat/completions surface (it even runs the agentic search loop server-side), but authenticates with X-Subscription-Token instead of Authorization: Bearer. Same story for Azure OpenAI (api-key).

What this does

Adds an auth field on targets, routes, and the flat config, decoupled from provider:

auth	Effect
bearer	Authorization: Bearer <key> (OpenAI/Ollama default)
api_key	x-api-key: <key> (Anthropic default)
{ header: "Name" }	arbitrary credential header
{ query: "param" }	key in the query string

routes:
  - pattern: "brave-*"
    provider: openai
    base_url: "https://api.search.brave.com/res"
    api_key: "env://BRAVE_API_KEY"
    auth: { header: "X-Subscription-Token" }

When auth is omitted it defaults to the provider's convention, so existing configs are unchanged (fully backward compatible). The three hardcoded auth sites now share a single apply_auth helper, and custom auth is propagated through to the /v1/models aggregator (UpstreamProvider.auth) so discovery does not silently fail for such targets.

Changes

• lib.rs: Auth enum, Provider::default_auth, TargetConfig::effective_auth, auth on TargetConfig/Route/flat config.
• providers/mod.rs: unified apply_auth (single source of auth-attachment logic).
• openai/anthropic transports + responses passthrough + models aggregator: all read apply_auth.
• config-schema.json: Auth $def + property on the three surfaces; vacuum rulesets regenerated.
• docs: dispatchers guide (config table, provider defaults, new "Custom credential headers" section), extensions reference; CHANGELOG entry.
• Cargo.lock: local SDK path deps resynced 0.7.0 -> 0.8.1 (build side effect, unrelated to the feature).

Testing

• 118 unit tests pass (8 new: Auth deserialization of both string and map forms, effective_auth provider defaults + explicit override, apply_auth for all four variants). Clippy clean (also fixed one pre-existing match_like_matches_macro warning in responses.rs).
• End-to-end proof against a compiled .bca + running gateway (patched wasm), with a WireMock upstream emulating Brave:
  • model: brave-research -> outbound x-subscription-token: <key>, no Authorization
  • model: gpt-4o -> outbound Authorization: Bearer <key> (default preserved)
  • both compose to /res/v1/chat/completions via base_url

Note

Whether Brave's grounding endpoint ultimately wants Bearer or X-Subscription-Token is a config choice now covered either way; no code change is needed to switch.