Cloudflare adapter should preserve oversized metadata for bodiless responses

[ChristianPavilonis]

Summary

The Cloudflare HTTP adapter applies its buffered response Content-Length cap before it evaluates HTTP response body semantics. As a result, a HEAD, informational (1xx), 204, 205, or 304 response may be rejected solely because metadata describes a representation larger than the buffering cap, even though no body needs to be copied.

Scope

• Evaluate request method and response status before the pre-flight and post-buffer size checks.
• Preserve valid metadata for bodiless responses, especially Content-Length on HEAD and 304.
• Add native-testable coverage for the shared decision logic where possible.

Context

Follow-up to #951; intentionally kept out of that Fastly-focused PR.