fix: index field in list which has upper maps

[wenchy]

Problem

When a list field has upper-level map containers, the index generation was not correctly handling the nested structure. The index key struct and lookup logic did not account for the map keys in the path from root to the indexed list field.

Solution

• Updated the index descriptor to properly track and include upper map keys when building index paths for list fields nested inside maps.
• Updated helper functions across all three language plugins (Go, C++, C#) to support the new nested map+list index structure.
• Added new test cases in index_conf.proto and corresponding generated test files to cover this scenario.

Changes

• internal/index/descriptor.go: Enhanced index descriptor to handle list fields with upper map containers.
• cmd/protoc-gen-go-tableau-loader: Updated helper, index, and ordered_index generators.
• cmd/protoc-gen-cpp-tableau-loader: Synced the same changes for C++ plugin.
• cmd/protoc-gen-csharp-tableau-loader: Synced the same changes for C# plugin.
• test/: Added new test proto definitions (Fruit5Conf, Fruit6Conf) and regenerated all loader code for Go, C++, and C#.