Fix b8 migration regressions, v2.3.5 milestone bugs, and convert to the property API

[sultann]

Summary

Follow-up to the bytekit -> b8 model migration on release/2.3.5. Fixes regressions the migration introduced, closes the three v2.3.5 milestone bugs, and converts the free plugin's own code off the deprecated getter/setter shims onto the new property API. The deprecated shims themselves are intentionally kept — master Pro (1.4.6) and the Legacy API add-on still call them.

All changes verified against the PHPUnit suite (89 tests green) and phpcs (clean).

Migration regressions

• Keys "filter by customer" silently broken — b8 dropped the old customer_id -> order_id IN (...) translation. Restored as a query_args filter on the Key model (honors the framework's empty-IN-is-ignored contract). +2 tests.
• enable_product never fired — the hook was wc_serial_numbers_key_insert, but b8 fires wc_serial_numbers_key_inserted and passes the model (not an id). Rewired and signature fixed. Removed three dead encrypt/decrypt action hooks the model now owns directly. +1 test.
• wcsn_order_replace_key counted failed saves as success — save() returns a truthy WP_Error on failure; now checks ! is_wp_error().

v2.3.5 milestone bugs

• Edit Serial Key redirects to Serial Key list page instead of opening edit form #527 Edit key redirects / fatals — Key::find() returns null for a missing id, so ! $key->exists() fataled. Null-guarded the edit screen lookup. +1 test.
• Bug: Legacy API Plugin Throws Critical Error on API Requests #531 Legacy API critical error — the REST activations field returned model objects (which JSON-encode to {}); b8 ignores the old ARRAY_A output arg. Now mapped through to_array() to restore the documented array contract. +1 test (incl. JSON round-trip).
• Bug: Export Returns Blank CSV When Exporting All Keys #532 Blank "Export All" CSV — master Pro passes product_id => 0 for "All Products", which b8 turns into WHERE product_id = 0 (no rows). Fixed free-side (Key::prepare_product_query drops a zero product_id) so unmodified master Pro works. +1 test.

Deprecated API -> property API conversion

Converted ~90 internal call sites across 11 files (REST, admin screens/list tables, templates, PDF integrations, core helpers) from $key->get_*() / $key->set_*() to property access ($key->serial_key, $key->status = ..., etc.). No behavior change; the deprecated methods remain for external consumers.

Pro compatibility

Master Pro 1.4.6's full call surface into the free plugin (wcsn_* functions, Key/Activation getters, where_query, the generator/duplicate filters, the REST activations contract) is satisfied. The only break was #532, now fixed free-side.

Closes #527, #531, #532.