[CT-81] network module has no test for a testnet/mainnet configuration mismatch

[mftee]

Overview

contract/src/module/network/mod.rs (179 lines) selects the Stellar network, but there's no evident startup-time check that the configured Horizon URL actually matches the declared network (e.g. STELLAR_NETWORK=mainnet pointed at a testnet Horizon URL) — this is exactly the class of misconfiguration that silently anchors documents on the wrong network.

Tasks

• Add a startup check that verifies the Horizon URL's network matches the declared STELLAR_NETWORK
• Fail startup with a clear error on mismatch
• Add a test for a deliberately mismatched configuration

Acceptance Criteria

• Starting the service with a mismatched network/Horizon-URL combination fails fast with a clear error

Notes for Contributors

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