Resolve outbound proxy routes explicitly (#34435)

## Why

System proxy discovery can block, and delegating fallback to each transport can repeat discovery or apply inconsistent environment proxy behavior.

## What changed

- Resolve unavailable system proxy decisions to an explicit environment proxy or direct route, including `NO_PROXY` settings and WebSocket-specific fallbacks.
- Add asynchronous system proxy resolution that uses cached decisions first and serializes blocking platform lookups on Windows and macOS.
- Preserve `NO_PROXY` handling for WebSocket connections, including HTTP and HTTPS proxies, and hash proxy cache keys on every platform.

## Testing

- Add coverage for explicit environment fallback, cached asynchronous resolution, and proxied or bypassed WebSocket connections.

GitOrigin-RevId: 3b7cf170dcfe639eec53c3c1514f92bfcf13e7e9

codex-rs/http-client/Cargo.toml

@@ -16,15 +16,13 @@ rustls-native-certs = { workspace = true }
 rustls-pki-types = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
+sha2 = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true, features = ["macros", "rt", "time", "sync"] }
 tracing = { workspace = true }
 tracing-opentelemetry = { workspace = true }
 zstd = { workspace = true }
 
-[target.'cfg(any(target_os = "windows", target_os = "macos"))'.dependencies]
-sha2 = { workspace = true }
-
 [target.'cfg(target_os = "macos")'.dependencies]
 system-configuration = { workspace = true }