朱子纯 / vibe-erp

refactor(metadata): HasExt marker + applyTo/parseExt helpers on ExtJsonValidator

Removes the ext-handling copy/paste that had grown across four PBCs
(partners, inventory, orders-sales, orders-purchase). Every service
that wrote the JSONB `ext` column was manually doing the same
four-step sequence: validate, null-check, serialize with a local
ObjectMapper, assign to the entity. And every response mapper was
doing the inverse: check-if-blank, parse, cast, swallow errors.

Net: ~15 lines saved per PBC, one place to change the ext contract
later (e.g. PII redaction, audit tagging, field-level events), and
a stable plug-in opt-in mechanism — any plug-in entity that
implements `HasExt` automatically participates.

New api.v1 surface:

  interface HasExt {
      val extEntityName: String     // key into metadata__custom_field
      var ext: String               // the serialized JSONB column
  }

Lives in `org.vibeerp.api.v1.entity` so plug-ins can opt their own
entities into the same validation path. Zero Spring/Jackson
dependencies — api.v1 stays clean.

Extended `ExtJsonValidator` (platform-metadata) with two helpers:

  fun applyTo(entity: HasExt, ext: Map<String, Any?>?)
      — null-safe; validates; writes canonical JSON to entity.ext.
        Replaces the validate + writeValueAsString + assign triplet
        in every service's create() and update().

  fun parseExt(entity: HasExt): Map<String, Any?>
      — returns empty map on blank/corrupt column; response
        mappers never 500 on bad data. Replaces the four identical
        parseExt local functions.

ExtJsonValidator now takes an ObjectMapper via constructor
injection (Spring Boot's auto-configured bean).

Entities that now implement HasExt (override val extEntityName;
override var ext; companion object const val ENTITY_NAME):
  - Partner (`partners.Partner` → "Partner")
  - Location (`inventory.Location` → "Location")
  - SalesOrder (`orders_sales.SalesOrder` → "SalesOrder")
  - PurchaseOrder (`orders_purchase.PurchaseOrder` → "PurchaseOrder")

Deliberately NOT converted this chunk:
  - WorkOrder (pbc-production) — its ext column has no declared
    fields yet; a follow-up that adds declarations AND the
    HasExt implementation is cleaner than splitting the two.
  - JournalEntry (pbc-finance) — derived state, no ext column.

Services lose:
  - The `jsonMapper: ObjectMapper = ObjectMapper().registerKotlinModule()`
    field (four copies eliminated)
  - The `parseExt(entity): Map` helper function (four copies)
  - The `companion object { const val ENTITY_NAME = ... }` constant
    (moved onto the entity where it belongs)
  - The `val canonicalExt = extValidator.validate(...)` +
    `.also { it.ext = jsonMapper.writeValueAsString(canonicalExt) }`
    create pattern (replaced with one applyTo call)
  - The `if (command.ext != null) { ... }` update pattern
    (applyTo is null-safe)

Unit tests: 6 new cases on ExtJsonValidatorTest cover applyTo and
parseExt (null-safe path, happy path, failure path, blank column,
round-trip, malformed JSON). Existing service tests just swap the
mock setup from stubbing `validate` to stubbing `applyTo` and
`parseExt` with no-ops.

Smoke verified end-to-end against real Postgres:
  - POST /partners with valid ext (partners_credit_limit,
    partners_industry) → 201, canonical form persisted.
  - GET /partners/by-code/X → 200, ext round-trips.
  - POST with invalid enum value → 400 "value 'x' is not in
    allowed set [printing, publishing, packaging, other]".
  - POST with undeclared key → 400 "ext contains undeclared
    key(s) for 'Partner': [rogue_field]".
  - PATCH with new ext → 200, ext updated.
  - PATCH WITHOUT ext field → 200, prior ext preserved (null-safe
    applyTo).
  - POST /orders/sales-orders with no ext → 201, the create path
    via the shared helper still works.

246 unit tests (+6 over 240), 18 Gradle subprojects.