Restore wrapped public payload converters

[tconley1428]

Direct calls to payload converters exposed by a data converter, workflow, or activity should apply the same transfer type conversion as SDK serialization. Revert #1875 and #1867 so these APIs consistently return the wrapped converter, including DataConverter.default.payload_converter and the System Nexus model conversion accessor.

This restores converter wrapping at construction, removes the separate internal converter accessors and unwrapping helper, and restores the tests from before those changes. Public API documentation and an unreleased changelog entry describe the resulting behavior.

Spec clarification: temporalio/features#909

Validation:

• 59 converter, System Nexus, and activity helper tests passed.
• 16 focused serialization context, dynamic activity (including process execution), and payload visitor tests passed.
• poe lint passed.
• git diff --check passed.