list_buckets panics via unimplemented!(); reachable by an unauthenticated non-GET request to /

Summary

SourceCoopRegistry::list_buckets is unimplemented!(), which panics rather than returning an error. It is reachable from a non-GET request to /.

Evidence

https://github.com/source-cooperative/data.source.coop/blob/main/src/source_api/registry.rs#L82-L88

async fn list_buckets(
    &self,
    _identity: &ResolvedIdentity,
) -> Result<Vec<BucketEntry>, ProxyError> {
    unimplemented!("Bucket listing is not supported")
}

The signature already returns Result<_, ProxyError>, so a panic is avoidable at no cost.

Reachability: multistore dispatches ListBuckets for a request to / (multistore-0.7.2/src/proxy.rs:874). IndexHandler intercepts / but only claims GET:

// src/handlers.rs
if req.method == http::Method::GET {
    Some(ProxyResult::json(200, format!("Source Cooperative Data Proxy v{}", VERSION)))
} else {
    None   // falls through to the gateway
}

So e.g. HEAD / or POST / falls through to the gateway and reaches list_buckets.

Impact

A panic in a Worker aborts the isolate and returns an opaque error to the caller, with no structured S3 error body. It is trivially reachable by an unauthenticated request. Not a data-integrity or disclosure risk, but it is an unauthenticated remote panic and it produces useless diagnostics.

Suggested fix

Return a proper S3 error instead of panicking — ProxyError::AccessDenied, or a NotImplemented-style error if one is a better fit for the S3 surface. Optionally have IndexHandler claim all methods on / so the fall-through cannot happen.

Note that implementing real bucket listing is a separate, larger question (it depends on the Role-ceiling work in the ADR set); this issue is only about not panicking.

Notes

Found while auditing the ADRs in #115 against the implementation.

Start in src/source_api/registry.rs at SourceCoopRegistry::list_buckets, then read the non-GET fall-through in src/handlers.rs and the ListBuckets dispatch noted in multistore-0.7.2/src/proxy.rs. Verify HEAD or POST requests to / no longer panic and instead return a structured S3 error; check whether existing request tests cover this path.