feat(contract): health endpoints, error sanitisation, concurrency limits, e2e tests [CT-46/57/58/60]

[Ummi-001]

Summary

Resolves four open issues for the contract/ Rust service (stellar-doc-verifier).

Closes #1059
Closes #1057
Closes #1056
Closes #1046

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Changes

#1059 [CT-60] Health & readiness endpoints

• GET /health/live — always 200 when the process is running, no dependency checks
• GET /health/ready — checks Redis + Horizon, returns 503 naming the failed dependency
• Horizon reachability result cached for 10 s to avoid health-poll load
• Reports service version (CARGO_PKG_VERSION) and Stellar network in readiness body
• Both endpoints are registered outside the rate-limited router group

#1057 [CT-58] Sanitized error responses

• New src/error.rs: stable ERR_* public error codes, ApiError type, new_request_id()
• Every error response carries a request_id field (UUIDv4) for server-side correlation
• No anyhow chain, internal path, upstream Horizon body, or type name reaches the client
• All handlers updated to use ApiError::bad_request, ApiError::upstream, ApiError::internal

#1056 [CT-57] Concurrency & resilience limits

• reqwest::Client built with 10 s timeout, pool max 10 idle/host, 90 s idle timeout
• Batch join_all bounded by a tokio::sync::Semaphore (BATCH_CONCURRENCY_LIMIT = 8)
• get_with_backoff() on StellarClient: retries HTTP 429 up to 3 times with exponential back-off, honours Retry-After header

#1046 [CT-46] End-to-end integration tests

• src/tests/integration.rs boots the full Axum router with httpmock + InMemoryCache
• Covers: health/live, health/ready (degraded), verify (mocked Horizon), cache-hit proof (mock call-count assertion), batch verify, empty/oversized batch validation, error sanitisation, revoke not-found
• 10 integration tests + tests in error.rs, health.rs

Testing

cargo test --all
cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings

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[mftee]

Excellent contract improvements! Health endpoints enable monitoring, error sanitization improves security, concurrency limits prevent abuse, and e2e tests ensure reliability.

[mftee]

Hi @Ummi-001, 👋

This PR has merge conflicts that need to be resolved before it can be merged. The following files have conflicts:

contract/src/error.rs
contract/src/lib.rs

Please resolve these conflicts in your branch and push the updates. Once resolved, the PR can be merged successfully.

Thank you!