Discover credentialed routes for managed proxy

[winston-openai]

Summary

• add credential route discovery/proxy endpoints to codex-backend-client
• fetch credentialed routes when a managed network proxy session starts
• leave actual MITM rerouting for the stacked follow-up PR

Validation

• just fmt
• cargo test -p codex-backend-client
• just bazel-lock-update
• just bazel-lock-check
• cargo test -p codex-core (blocked by existing untouched turn_extension_data errors in core/src/tasks/mod.rs)

[github-actions]

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