## Summary
Stacked on #26706.
Adds the shared auth/system-proxy contract that later platform resolver
PRs plug into. This PR moves Codex-owned auth and startup HTTP clients
through a common route-aware boundary, but does not yet add Windows or
macOS system proxy resolution.
The default path remains unchanged when `respect_system_proxy` is absent
or disabled.
## Implementation
- Adds `codex-client/src/outbound_proxy.rs` with the shared
route-selection model:
- `OutboundProxyConfig`;
- `ClientRouteClass`;
- `RouteFailureClass`;
- `build_reqwest_client_for_route`.
- Preserves the existing reqwest/default-client behavior when no route
config is supplied.
- Uses the fixed MVP routing policy when route config is supplied:
platform system/PAC/WPAD discovery, then explicit env proxy variables,
then direct connection.
- Keeps platform-specific system discovery behind the shared client
boundary. This PR provides the contract and fallback behavior; later
resolver PRs plug in Windows and macOS discovery.
- Adds `login::AuthRouteConfig` so auth call sites depend on a small
policy type instead of platform resolver details.
- Maps the resolved `Config.respect_system_proxy` boolean into
`AuthRouteConfig` for auth-owned clients.
- Wires the route config through browser login, device-code login,
access-token login, login status, logout/revoke, token refresh, API-key
exchange, app-server account login, TUI/app startup, cloud-config
bootstrap, cloud tasks, plugin auth, and exec startup config loading.
## End-user behavior
- No behavior changes by default.
- When `respect_system_proxy = true`, auth-owned clients opt into the
shared route-aware client path.
- On platforms without a resolver implementation in this PR, system
discovery is unavailable and the route-aware path falls back to explicit
env proxy handling, then direct connection.
- Custom CA handling remains separate from proxy route selection and
still runs through the shared client builder.
- No proxy URLs, PAC contents, or resolved platform details are exposed
through the public config surface introduced here.
## Tests
Adds or updates coverage for:
- preserving default auth-client fallback behavior when no route config
is provided;
- injected environment-proxy fallback without mutating process
environment;
- existing login-server E2E flows using explicit `auth_route_config:
None` to guard unchanged default behavior;
- updated auth manager, login, logout, cloud-config, startup, and
plugin-auth call sites passing route config explicitly.