Hide rpc headers when debug tracing the network.

[fnando]

What

$ STELLAR_RPC_HEADERS=a:1 STELLAR_RPC_URL=http://localhost:8000/rpc STELLAR_NETWORK_PASSPHRASE="Standalone Network ; February 2017" stellar contract invoke --id hello --very-verbose -- hello --from fnando
2026-03-10T18:15:01.317066Z DEBUG soroban_cli::upgrade_check: start upgrade check
2026-03-10T18:15:01.317055Z TRACE soroban_cli::commands::contract::invoke: network=Network { rpc_url: "http://localhost:8000/rpc", rpc_headers: [("a", "<concealed>")], network_passphrase: "Standalone Network ; February 2017" }
2026-03-10T18:15:01.317228Z DEBUG soroban_cli::upgrade_check: finished upgrade check
2026-03-10T18:15:01.317401Z TRACE soroban_cli::get_spec: network=Network { rpc_url: "http://localhost:8000/rpc", rpc_headers: [("a", "<concealed>")], network_passphrase: "Standalone Network ; February 2017" }
...

Why

https://hackerone.com/reports/3596218

Known limitations

N/A

[Copilot]

Pull request overview

This PR prevents sensitive RPC header values (e.g., API keys / Authorization tokens) from being exposed in trace/debug logs by customizing how Network is formatted for Debug.

Changes:

• Remove the derived Debug implementation from Network.
• Add a manual Debug impl that prints RPC header values as "<concealed>".
• Add a unit test asserting the redacted debug output format.