Test/coverage health indexer

[Freezyyy-arch]

Closes
Closes #33
Closes #34
Closes #35
Closes #36

Summary

Issue 1 (
mod.rs
): Two tests — health_returns_ok_body asserts GET /health returns {"status":"ok"}; index_lists_all_endpoints asserts GET / includes all 8 documented routes in its endpoints array.

Issue 2 (
mod.rs
): refresh_builds_expected_snapshot — spins up a wiremock server returning canned XDR-encoded responses for get_all_assets, get_metadata, get_allowlist, and dividends, then asserts the snapshot has the right asset count, symbol, and ledger number.

Issue 3 (
mod.rs
): good_asset_survives_partial_error — seeds a good snapshot with asset A, then runs a second refresh where asset B's metadata call returns an RPC error; asserts the snapshot still holds the original asset A and the ledger hasn't advanced.

Issue 4 (
mod.rs
): failing_refresh_preserves_previous_snapshot — seeds a snapshot, then makes the RPC return HTTP 500; asserts the snapshot is completely unchanged after the failed refresh.

[drips-wave]

@Freezyyy-arch Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits