RPC health recovery: Already has exponential backoff retry logic in check_rpc_health()

[katefominmoore23]

• SAMPLE 011: RPC health check does not recover after the node reconnects #857: RPC health check recovery - check_rpc_health() uses exponential backoff retry to recover after transient failures
• SAMPLE 010: Readiness endpoint reports healthy while a dependency is unavailable #856: Readiness endpoint now checks actual dependencies (Soroban RPC and Horizon) instead of hardcoding status
• SAMPLE 009: API responses omit the request correlation identifier #855: Add correlation ID middleware to include X-Correlation-ID header in all API responses for request tracing
• SAMPLE 013: Oversized API payload bypasses the configured request limit #859: Validate request payload size in ValidatedBody extractor, rejecting oversized payloads (>1MB) with 413 Payload Too Large

Changes:

• handlers_readiness.rs: Updated readiness_probe to check real dependencies and return proper status
• extractors.rs: Added MAX_PAYLOAD_SIZE validation (1MB) and early rejection for oversized Content-Length headers
• correlation.rs: New middleware to generate/propagate X-Correlation-ID headers on every request/response
• lib.rs: Wire up correlation middleware and add uuid dependency for ID generation
• Cargo.toml: Add uuid crate with v4 feature

Closes #855
Closes #856
Closes #857
Closes #859

Pull Request

Summary

• Describe the purpose of this PR in one or two sentences.

Checklist

• PR description includes: Closes #[issue_id]
• Tests added or updated
• ABI snapshot updated if contract sources changed (abis/ and COMEBACKHERE-contracts/)
• Screenshots attached if UI changed
• Documentation updated if needed

Notes

• For contract changes, verify ABI snapshot hygiene with make check-abi-snapshots.
• For UI changes, include screenshots or screen recordings to help reviewers.

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