Implement HTTP 103 Early Hints for Swagger UI critical resources

[Copilot]

The main page loads four blocking resources (CSS, 2x JS bundles, JSON spec) serially. HTTP 103 Early Hints allows browsers to discover and fetch these resources while the server prepares the HTML response from KV store.

Changes

• Modified swagger_ui_html() to send HTTP 103 response with Link preload headers before the main response
• Added preload directives for:
  • swagger-ui.css (as=style)
  • swagger-ui-bundle.js (as=script)
  • swagger-ui-standalone-preset.js (as=script)
  • openapi-spec.json (as=fetch, crossorigin)

Implementation

fn swagger_ui_html(mut resp: Response) -> Result<Response, Error> {
    let early_hints = Response::from_status(103)
        .with_header("Link", "</static-assets/swagger-ui.css>; rel=preload; as=style")
        .with_header("Link", "</static-assets/swagger-ui-bundle.js>; rel=preload; as=script")
        .with_header("Link", "</static-assets/swagger-ui-standalone-preset.js>; rel=preload; as=script")
        .with_header("Link", "</static-assets/openapi-spec.json>; rel=preload; as=fetch; crossorigin");
    
    early_hints.send_to_client();
    
    // ... continue with normal response
}

Impact

Reduces time-to-interactive by parallelizing KV store lookup with resource fetching. Most beneficial on high-latency connections where round-trip time dominates.

Original prompt

Review the following implementation for early hints.

use fastly::http::StatusCode;
use fastly::{Error, Request, Response};

#[fastly::main]
fn main(_req: Request) -> Result<Response, Error> {
    println!("running version {}", std::env::var("FASTLY_SERVICE_VERSION")
          .unwrap_or_else(|_| "<version>".to_string()));

    let early_hints = Response::from_status(103)
        .with_header("Link", "</style>; rel=preload; as=style")
        .with_header("Link", "</script.js>; rel=preload; as=scrypt");

    early_hints.send_to_client();

    Ok(Response::from_status(StatusCode::OK)
        .with_body("Here's the real HTTP body!"))
}

You need to review the site https://http.edgecompute.app/ and identify where early hints may be implemented to improve the site performance. Propose an implementation in main.rs

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