[Ruby FFI] Map#merge always raises when merging Hash or Map values

Summary

The FFI implementation of Google::Protobuf::Map#merge calls private helpers with explicit receivers. Ruby rejects both calls: Map input reaches other.descriptor, and all accepted inputs reach internal_dup.internal_merge_into_self, producing NoMethodError instead of a merged copy.

Proposed correction

Use the existing private reflective access consistently for descriptor and internal_merge_into_self. Candidate: 00c482150dab1ad152135ab860632c8ae2a11c26 on current main (5b1c20741838b8359193b97895cb0ff35b4ecf79).

The isolated source-built FFI model covers Hash and compatible Map merges, result independence, and unchanged inputs. The cumulative source-built FFI suite passes 329 tests /598,403 assertions with no failures/errors and nine documented omissions; native passes 332 /556,948 with three omissions.

I used an AI coding assistant during source review and verification. I am opening an issue first because CONTRIBUTING asks for tests and maintainer agreement; this dependency audit is not permitted to modify repository tests and no CLA was signed as part of it.

Start at the Ruby FFI implementation of Google::Protobuf::Map#merge and inspect its private helper calls, especially descriptor and internal_merge_into_self. Use the isolated source-built FFI model to verify Hash and compatible Map merges, result independence, and unchanged inputs, then confirm the reported source-built and native suite results.