Make Http.TransportErr match host error behavior

[lukewilliamboswell]

Context

InternalHttp.TransportErr declares:

[Timeout, NetworkError, BadBody, Other(List(U8))]

in platform/InternalHttp.roc:12.

The Rust host constructs Timeout, BadBody, and Other, but never constructs NetworkError. Every error returned by client.request(request).await is converted to Other(err.to_string().as_bytes()) at src/http.rs:167-170.

The connection-refused integration scenario currently demonstrates this behavior: it produces HttpErr(Other(... "client error (Connect)" ...)).

This is related to #142, which was closed because error details are now retained in Other. The remaining problem is that the public union still advertises an unreachable NetworkError variant.

Suggested direction

Choose and document one consistent contract:

1. classify DNS/connect/TLS/socket failures as NetworkError(details), potentially changing its payload; or
2. remove NetworkError and define when Other is used.

Prefer stable structured categories over matching Hyper's display strings. Preserve useful diagnostic details without making callers parse text to identify ordinary network failures.

Acceptance criteria

• Every public TransportErr variant has a documented, reachable meaning, or unreachable variants are removed.
• Connection refused, DNS failure, TLS failure, timeout, invalid response body, and request-construction failure are mapped intentionally.
• Tests assert error variants, not only that an HTTP request failed.
• Documentation and examples reflect the final error shape.
• Any API change is included in the release bump check.