Add MpoolGetConfig RPC and use runtime RBF ratio in mpool replace --auto

[coderabbitai]

Background

The mpool replace --auto command currently hardcodes REPLACE_BY_FEE_RATIO_DEFAULT when computing the RBF minimum premium floor via compute_rbf. If a node is configured with a non-default replace_by_fee_ratio, the auto replacement can underbid and be rejected with GasPriceTooLow.

Required work

1. Implement MpoolGetConfig RPC — expose the node's mpool configuration (including replace_by_fee_ratio) over the API, matching the Lotus MpoolGetConfig interface.

2. Use runtime RBF ratio in mpool replace --auto — in src/cli/subcommands/mpool_cmd.rs, retrieve the replace_by_fee_ratio from MpoolGetConfig before calling compute_rbf, and pass the runtime value instead of REPLACE_BY_FEE_RATIO_DEFAULT so the auto replacement floor matches the running node's mpool configuration.

References

• Lotus MpoolGetConfig API docs: https://github.com/filecoin-project/lotus/blob/master/documentation/en/api-methods-v1-stable.md#MpoolGetConfig
• Identified in PR Add nonce-fix and replace mpool tools #7084 (comment Add nonce-fix and replace mpool tools #7084 (comment))

/cc @sudo-shashank