Add custom headers support for models.json

Fixes #39

- Added headers field to Model type (provider and model level)
- Model headers override provider headers when merged
- Supported in all APIs:
  - Anthropic: defaultHeaders
  - OpenAI (completions/responses): defaultHeaders
  - Google: httpOptions.headers
- Enables bypassing Cloudflare bot detection for proxied endpoints
- Updated documentation with examples

Also fixed:
- Mistral/Chutes syntax error (iif -> if)
- process.env.ANTHROPIC_API_KEY bug (use delete instead of = undefined)

packages/ai/src/providers/anthropic.ts

@@ -288,11 +288,12 @@ function createClient(
 			accept: "application/json",
 			"anthropic-dangerous-direct-browser-access": "true",
 			"anthropic-beta": "oauth-2025-04-20,fine-grained-tool-streaming-2025-05-14",
+			...(model.headers || {}),
 		};
 
 		// Clear the env var if we're in Node.js to prevent SDK from using it
 		if (typeof process !== "undefined" && process.env) {
-			process.env.ANTHROPIC_API_KEY = undefined;
+			delete process.env.ANTHROPIC_API_KEY;
 		}
 
 		const client = new Anthropic({
@@ -309,6 +310,7 @@ function createClient(
 			accept: "application/json",
 			"anthropic-dangerous-direct-browser-access": "true",
 			"anthropic-beta": "fine-grained-tool-streaming-2025-05-14",
+			...(model.headers || {}),
 		};
 
 		const client = new Anthropic({