Backend + HTTP client traits

Description

Implement PlatformBackend and PlatformHttpClient backed by Fastly SDK. Introduce PlatformResponse with backend_name: Option<String> for upstream correlation. Abstract send/send_async behind traits. Integration send calls deferred to PR 13.

EdgeZero ProxyClient relationship

Before this PR merges, file an EdgeZero issue to generalize ProxyClient into an HttpClient trait supporting both synchronous proxy-style sends and the async fan-out pattern (send_async + select). The trusted-server PlatformHttpClient Fastly impl works independently until the generalized EdgeZero trait lands, at which point the Fastly impl swaps to implementing the EdgeZero trait.

Done when

PlatformResponse with backend_name
send/send_async abstracted
EdgeZero issue filed for ProxyClient → HttpClient generalization
Per-PR gates pass

Blocked by

PR 2

References

docs/internal/EDGEZERO_MIGRATION.md — Phase 1, PR 6
Part of EdgeZero Integration #480

Address platform layer review feedback

Flagged in PR #545 review: PlatformBackendSpec and BackendConfig have overlapping fields (scheme, host, port, certificate_check, first_byte_timeout). Every ensure call allocates owned Strings that are immediately borrowed. Unifying or sharing the type is in scope for this issue.