docs: Clarify that only Coder tokens work with AI Bridge authentication

Problem

The AI Bridge client config authentication section mentions "session token or API key" but doesn't explicitly state that only Coder-issued tokens are accepted. This could lead to confusion where users attempt to use provider-specific API keys (OpenAI, Anthropic, etc.) which will not work.

Proposed Changes

  1. Change "API key" to "Coder API key" throughout the Authentication section to be explicit
  2. Add a note clarifying that provider-specific API keys will not work with AI Bridge

Current Text

Instead of distributing provider-specific API keys (OpenAI/Anthropic keys) to users, they authenticate to AI Bridge using their Coder session token or API key

Proposed Text

Instead of distributing provider-specific API keys (OpenAI/Anthropic keys) to users, they authenticate to AI Bridge using their Coder session token or Coder API key

Note: Only Coder-issued tokens are accepted. Provider-specific API keys (such as OpenAI or Anthropic keys) will not work with AI Bridge.


Created on behalf of @dannykopping