http-client: add route-aware redirect handling

[bolinfest]

Why

Redirects are new requests whose destinations may require different PAC or OS proxy decisions. Leaving redirect handling inside one reqwest client would reuse the original route, while a custom loop also has to preserve reqwest's timeout, method/body, Referer, and credential-stripping behavior.

What changed

• Follow supported redirects through RouteAwareClientPool so every hop resolves its own complete URL.
• Preserve one total request timeout across proxy resolution, transport work, and redirect hops.
• Match reqwest's redirect method/body rules and sanitized Referer behavior.
• Strip authorization, cookie, and proxy credentials whenever scheme, host, or effective port changes.
• Reject unsupported redirect schemes and bound redirect chains.
• Add exact-route, per-hop proxy, credential, timeout, and no-diagnostics privacy coverage.

Review guide

1. http-client/src/route_aware_redirect.rs contains the redirect transformation and origin rules.
2. The send loop in route_aware_client_pool.rs applies those rules while re-resolving each hop.
3. outbound_proxy_tests.rs, route_aware_redirect_tests.rs, and the integration test cover the behavior and privacy invariants.

Validation

• just test -p codex-http-client
• Focused privacy and redirect-routing regression tests.

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• core-plugins: route curated startup sync through HTTP client factory #31924
• app-server-daemon: route updater download through HTTP client factory #31828
• cloud-tasks: route environment requests through HTTP client factory #31825
• backend-client: route requests through HTTP client factory #31821
• -> http-client: add route-aware redirect handling #31918
• http-client: add safe route-aware request pool #31917
• http-client: make proxy route fallback explicit #31916