feat: support ao.unset sentinel for patch field removal

[Lucifer0x17]

Summary

• Adds convert_unset_values/1 to dev_json_iface that recursively converts <<"__ao-unset__">> sentinel strings to the unset atom during JSON result deserialization
• Applied in both preprocess_results (outbox messages) and json_to_message (patches path)
• dev_message:set/3 already handles the unset atom by removing those keys -- no changes needed there

Problem

When AOS processes use patch@1.0 to cache state for frontend HTTP access, removing a field from a nested map is impossible. Lua nil deletes the key from the table before serialization, so the patch system never learns the field should be removed. The deep merge in dev_message:set preserves old keys that are absent from the new patch.

Solution

A sentinel value "__ao-unset__" that survives Lua table serialization and JSON encoding. At the HB boundary (dev_json_iface), it is converted to the unset atom that dev_message:set already handles.

Requires a companion change in the AOS repo: ao.unset = "__ao-unset__" in ao.lua.

Usage

ao.unset = "__ao-unset__"  -- defined once in ao.lua

-- Remove cherry from nested map, update apple, keep banana
Send({device="patch@1.0", prices={apple="200", cherry=ao.unset}})

-- Clear a top-level key (set to empty map)
Send({device="patch@1.0", mode={}})