feat: support custom headers in customai resource type

[centdix]

Summary

Allow users to add custom HTTP headers to the customai resource type. When the hub updates the resource type to include a headers field (a JSON object mapping header names to values), those headers will be sent on every AI request for that resource.

Changes

• Add headers: HashMap<String, String> field (with #[serde(default)]) to AIStandardResource and ProviderResource structs
• Thread custom headers through AIRequestConfig and apply them in the API proxy's prepare_request()
• Apply resource custom headers in the worker's build_http_request closure for AI agent execution

Test plan

• Verify existing AI resources without a headers field still work (backwards-compatible via #[serde(default)])
• Create a customai resource with "headers": {"X-Custom": "test"} and verify the header is sent on AI requests
• Confirm custom headers from the resource are applied after env-var AI_HTTP_HEADERS (resource headers take precedence)

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