test: add scheduled Stellar Testnet E2E integration suite

[Wilfred007]

Mocked RPC tests cannot catch live-network drift (protocol upgrades, ledger timing, RPC behaviour, fee surges, Friendbot outages) or indexer ingestion regressions. Add a live testnet suite and a scheduled workflow:

• backend/e2e/testnet/network-health: RPC health, passphrase/protocol version, ledger close-time budget, RPC/Horizon skew, fee stats, Friendbot.
• backend/e2e/testnet/stream-lifecycle: create_stream -> withdraw ->
  cancel_stream on a deployed contract with Friendbot-funded accounts and the native XLM SAC, then runs the real SorobanEventWorker against Postgres and asserts events are ingested with matching hashes/ledgers.
• .github/workflows/testnet-e2e.yml: every 6h (and on dispatch) builds and deploys a fresh contract with an ephemeral deployer, runs the suite, and opens/updates a testnet-e2e-failure issue on scheduled failures.

The suite has its own vitest config and is excluded from npm test.

Description

Closes #1454
Closes #1448
Closes #1447
Closes #1446

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #

Changes Made

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

Breaking Changes

Breaking Changes:

Migration Guide:

Screenshots/Demo

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Updated Postman/Hoppscotch API collections if routes changed
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have checked for breaking changes and documented them if applicable

Additional Notes