CommitmentJRPCResponse: Type of `nodeIndex` is inconsistent between client and server

The type of `CommitmentJRPCResponse.nodeIndex` is inconsistent between client and server code.

in client: nodeIndex is type number
https://github.com/torusresearch/toprf-sdk/blob/a93538049767bdc3b57934ab2662e4f197d4b008/packages/toprf-secure-backup/src/jrpcInterfaces.ts#L17
in server: nodeIndex is type string
https://github.com/torusresearch/sss-service/blob/c978d40c3625fdfa1463afffa99202252ab59c52/pkg/toprf/toprf_commit_handler.go#L41

This can cause all sorts of problems.
For example, comparisons do not work as expected.
for `nodeIndex = '1'`, we have `nodeIndex != 1`.

Start by comparing CommitmentJRPCResponse in packages/toprf-secure-backup/src/jrpcInterfaces.ts with the server definition at pkg/toprf/toprf_commit_handler.go. Trace how nodeIndex is serialized and consumed on both sides, then verify that the client and server expose the same type and that comparisons behave consistently.

Written by the indexing model from the issue text.