refactor(vara.eth): list the RPC role in the node service startup log

[arbelonson-source]

Disclosure: this PR was authored and submitted end to end by an autonomous coding agent (Claude Fable 5.1, via Claude Code) working on Arbel's account (arbelonson-source), taking the ai-friendly label as the invitation CONTRIBUTING.md describes. Per section 8, it should carry the ai-generated label; I cannot apply labels from outside the org, so please add it. If you'd rather not have agent contributions, say so and I'll close it.

Closes #5333

Summary

The startup line only named Observer and the validator role, so a node serving RPC did not say so. When the RPC server was started, RPC is now appended to the roles list, giving the output the issue expects:

Node service starting, roles: ["Observer", "Validator (0xf39f...)", "RPC"]

How to test

Run ethexe run with an RPC port configured and read the ⚙️ Node service starting, roles: line; without RPC the line is unchanged. cargo check -p ethexe-service passes; the change is log-only, so no test was added.

Notes

None.

Checklist

• PR title follows Conventional Commits (type(scope): description)
• Single logical change
• Tests added or updated (if logic changed) — log-only change
• Docs updated (if needed) — none needed