feat: redefine HTTP status codes and add internal status header

[ruslanti]

No description provided.

[Copilot]

Pull request overview

This PR introduces a new internal failure classification mechanism by emitting an X-CDN-Internal-Status response header and documenting a 3000–3999 internal status code range to disambiguate runtime failure reasons behind existing 5xx status codes.

Changes:

• Adds internal status code constants (3000–3999) and a new X-Cdn-Internal-Status header constant.
• Extends error mapping to return an internal status code and attempts to attach it to error responses.
• Updates executor HTTP tests and README documentation to cover/describe the new header.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
crates/http-service/src/lib.rs	Adds internal status constants and attempts to set X_CDN_INTERNAL_STATUS on error responses.
crates/http-service/src/executor/http.rs	Updates tests to assert presence/value of the new internal status header for timeout/OOM.
README.md	Documents internal status code meanings and the header contract.

Comments suppressed due to low confidence (1)

crates/http-service/src/lib.rs:365

• Because app_res_headers(cfg) is folded into the response builder after X-CDN-Internal-Status is set, a configured app response header with the same name could append a second value and make the internal status ambiguous for downstream consumers. If X-CDN-Internal-Status is meant to be authoritative, set/overwrite it after applying app headers (or insert it into the built response’s HeaderMap with insert and optionally remove any existing values).

                let builder = hyper::Response::builder()
                    .status(status_code)
                    .header(X_CDN_INTERNAL_STATUS, internal_code);
                let res_headers = app_res_headers(cfg);
                let builder = res_headers
                    .iter()
                    .fold(builder, |builder, (k, v)| builder.header(k, v));

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