fix: preserve option_type_id across syncs + don't clobber attrs with empty values

[mageaustralia]

Fixes #3, #4.

What

1. CustomOptionsTrait — value-level title merge: incoming option values are matched to existing values by title within their option, and the destination option_type_id is preserved in the payload. Core's product-option save handler then UPDATEs the row in place instead of delete+recreate, so option_type_ids stay stable across syncs.

2. Default options_mode flipped from 'replace' to 'merge' (3 call sites). 'replace' is still available when explicitly requested via _entity_options['options_mode'].

3. Guard against empty overwrites of attributes with non-empty defaults: in the custom-attribute write loop, if the incoming value is empty/null AND the attribute's default_value is non-empty, the write is skipped (destination value preserved). Closes Empty source options_container overwrites destination → frontend options hidden #4 — generalises to any UI-gating attribute with a default.

Why

A single sync that carried custom_options was orphaning the entire dependent-options config on a real install (~450 products in one go) — every value's option_type_id was recreated, breaking historical order/quote item options and 3rd-party modules keyed off those IDs. The options_container clear was hiding product option dropdowns site-wide for any product whose source payload had an empty value.

Test plan

• Trigger a sync for a product with custom options (e.g. via datasync:incremental after marking the product in datasync_change_tracker).
• Verify catalog_product_option_type_value.option_type_id values for that product are unchanged after the sync (snapshot before/after).
• Verify any 3rd-party module rows referencing those option_type_ids (e.g. Pektsekye-style optiondependent_value.option_type_id) remain valid after the sync.
• For the empty-default guard: trigger a sync for a product whose source has options_container = '' and whose destination has a non-empty value — confirm the destination value is preserved.

Notes

• Backwards-compatible across all option modes: replace deletes existing options first → new options have no $option->getId() → the existing-by-title map is empty → behaviour identical to before. merge sets option_id from title match → getId() returns the destination id → existing values fetched → title-keyed preserve kicks in. append adds only new → no existing match → same as replace path for the value step.
• _createOptionValues() is dead code (never called from this flow — $option->setData('values', …) + $option->save() goes through core's own value handler). Left alone here; a separate cleanup PR can remove it.