Cloudflare adapter: add native test coverage for the wasm-only HTTP client (redirect/cap/header logic) #829

Description

Follow-up from PR #643 review (inline comment).

The wasm32-only CloudflareHttpClient (execute/select, redirect handling, response-body cap, and hop-by-hop header stripping) has no test target — CI only cargo checks it for wasm32-unknown-unknown, never exercises it. This is how the round-7 redirect-follow SSRF bug went unverified.

Not blocking for PR #643 (the logic is now reviewed and the fixes are in), but this path needs real coverage.

What to do

• Extract the redirect, body-cap, and header-stripping logic out of the wasm-only CloudflareHttpClient into native-testable free functions, as the Axum and Fastly adapters already do for their HTTP clients.
• Add native unit tests exercising:
  • manual redirect handling / per-hop allowlist behavior (no auto-follow),
  • the 10 MiB response-body cap (Content-Length pre-check + post-buffer check),
  • hop-by-hop header stripping (is_hop_by_hop_response_header + Connection-token list).

Done when

• Redirect / cap / header-stripping logic lives in native-testable free functions.
• Native unit tests cover redirect, body-cap, and header-stripping behavior.
• Tests run in CI on a native target (not only cargo check on wasm).

Affected area

Cloudflare runtime / CI / Tooling