test(e2e): add nightly testnet smoke suite for Horizon and RPC flows (#1002)

[yodakkak]

Closes #1002

Summary

• Added scripts/testnet-smoke-suite.mjs exercising fresh testnet account funding, payment/trustline, path payment, Soroban contract deploy/invoke, and event read flows.
• Added �ssertNoMainnetAccess guard blocking any mainnet Horizon/RPC/passphrase configuration, and isTransientNetworkError +
  unWithTransientRetry separating transient network retries from assertion failures.
• Added .github/workflows/nightly-testnet-smoke.yml (10-minute budget, deduplicated tracking issue on failure) and unit tests in ests/unit/scripts/testnet-smoke-suite.test.js.

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