[EPIC] Remove all pub field accessor.

### What's the feature are you trying to implement?

With more and more features introduced, we found one common requirement: we need to ensure that when a struct instance has been built, we need to ensure that it's valid. Some structs expose pub field accessors directly, which makes maintaining the guarantee infeasible. We need to remove all pub accessors of those struct.

### Willingness to contribute

None

No files, tests, or entry points are named. Start by inventorying the structs that expose public field accessors and determine the scope of the epic from the existing Rust APIs. Done means the targeted public accessors have been removed so struct validity can be maintained after construction.

Written by the indexing model from the issue text.