Add monero-rpc-pool test coverage

[louiss72]

Summary

• add unit tests for monero-rpc-pool config, network parsing, server URL formatting, and node health stats helpers
• add an integration test that starts the pool on a random port and verifies the /stats endpoint returns the expected health payload
• add dev-dependencies needed for the new integration coverage

Verification

• git diff --check
• cargo test -p monero-rpc-pool could not be run locally because this environment does not have cargo/rustc installed

Refs #801

Preferred payout asset if accepted: XMR.

[Einliterflasche]

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