feat: improve RPC error logging to show detailed provider messages

[Copilot]

Problem

When RPC providers like Alchemy return errors (especially HTTP 429 rate limit errors), only the status code was logged without the detailed error message from the response body. This made debugging provider issues difficult.

Example of current logging:

HTTP error: 429 Too Many Requests

What the provider actually returns:

{
  "error": {
    "code": 429,
    "message": "Monthly capacity limit exceeded. Visit https://dashboard.alchemy.com/settings/billing to increase your capacity"
  }
}

Solution

Modified RelayError::RpcError to extract and display the detailed error message from JSON-RPC error responses when available.

New logging format:

RPC error: Monthly capacity limit exceeded. Visit https://dashboard.alchemy.com/settings/billing to increase your capacity (code: 429)

Changes

• Modified src/error/mod.rs:

  • Changed RelayError::RpcError variant from #[error(transparent)] to use custom formatting
  • Added format_rpc_error() function that extracts error.message and error.code from JSON-RPC responses
  • Falls back to default formatting for transport-level errors (network issues, etc.)

• Added comprehensive tests:

  • Test for Alchemy rate limit errors (HTTP 429)
  • Test for generic RPC execution errors
  • Verified error messages contain both the detailed message and error code

Benefits

✅ Better debugging: Developers immediately see why the provider failed
✅ Actionable information: Error messages often include links to provider dashboards
✅ Consistent format: All RPC errors show detailed information when available
✅ No breaking changes: Falls back gracefully for errors without detailed responses

Examples

Alchemy Rate Limit

Before: HTTP error: 429 Too Many Requests
After:  RPC error: Monthly capacity limit exceeded. Visit https://dashboard.alchemy.com/settings/billing... (code: 429)

Execution Reverted

Before: HTTP error: 400 Bad Request
After:  RPC error: execution reverted: insufficient balance for transfer (code: -32000)

This change directly addresses the need to avoid situations where the relay goes down due to provider issues and we cannot figure out the reason from the logs.

Fixes #[issue number]

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• arb1.arbitrum.io
  • Triggering command: /home/REDACTED/work/relay/relay/target/debug/deps/relay-0116f8e29f20dbc0 (dns block)
• eu-central-mainnet.rpc.ithaca.xyz
  • Triggering command: /home/REDACTED/work/relay/relay/target/debug/deps/relay-0116f8e29f20dbc0 (dns block)
• mainnet.optimism.io
  • Triggering command: /home/REDACTED/work/relay/relay/target/debug/deps/relay-0116f8e29f20dbc0 (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Improve error logging for Alchemy rate limit (429) errors</issue_title>
<issue_description>## Problem

Alchemy rate limit errors (HTTP 429) only log the status code, not the detailed error message from the response body.

This is hard to debug.

Current log:

HTTP error: 429 Too Many Requests

Alchemy response body contains:

{"error": {"code": 429, "message": "Monthly capacity limit exceeded. Visit https://dashboard.alchemy.com/settings/billing..."}}

Location: https://github.com/ithacaxyz/relay/blob/main/src/chains.rs#L435

Parse and log the error.message field from JSON-RPC error responses.</issue_description>

<agent_instructions>what we want to do is to avoid a situation where it goes down because of a provider and we cannot figure out a reason</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #1444

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