test(vendors,po): add co-located unit tests for VendorsService and PurchaseOrdersService

[Gracora]

Summary

Closes #1288
Closes #1289
Closes #1290
Closes #1291

Adds co-located vendors.service.spec.ts (11 tests) and purchase-orders.service.spec.ts (22 tests) covering every public method in both services, replacing the single happy-path assertions in ibinola.spec.ts with focused, discoverable coverage.

Why

The only tests touching VendorsService and PurchaseOrdersService were two it() blocks in backend/src/ibinola.spec.ts — a file bundling unrelated modules. Coverage missed VendorsService.update(), delete() with the open-PO guard, and the full PO status state machine (submit, approve, receive, cancel). Co-locating tests with the modules they exercise follows the project convention and makes failures immediately attributable.

What was built

backend/src/vendors/vendors.service.spec.ts:

Test group	What it covers
findAll	Paginated results, search filter, totalPages calculation
findById	Returns vendor; NotFoundException for missing id
create	Creates vendor record
update	Updates existing; NotFoundException for missing id
delete	Deletes with no open POs; NotFoundException; BadRequestException when open POs exist; allows delete when all POs are RECEIVED

backend/src/purchase-orders/purchase-orders.service.spec.ts:

Test group	What it covers
findAll	Paginated results, search filter on poNumber
findById	Returns PO with line items; NotFoundException
create	Auto-generated poNumber; with line items; initial DRAFT status
update	Updates DRAFT PO; NotFoundException; BadRequestException for non-DRAFT; replaces line items
submit	Submits DRAFT; BadRequestException for non-DRAFT; NotFoundException
approve	Approves SUBMITTED; records approver; BadRequestException for non-SUBMITTED
receive	Receives APPROVED; BadRequestException for non-APPROVED
cancel	Cancels DRAFT; cancels SUBMITTED; BadRequestException for APPROVED; BadRequestException for RECEIVED; NotFoundException

No existing files modified — only additions, so regression risk is low.

Acceptance criteria coverage

• Both modules have real, co-located coverage beyond their single existing happy-path tests (11 + 22 = 33 new tests across two spec files)

Test plan

• npm test in backend/ — 33/33 passing (all new tests for this feature)

Env vars / Notes

No new environment variables. Tests use lightweight Jest mocks for TypeORM repositories — no database connection required.

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[yusuftomilola]

No merge conflicts with main. Good co-located unit test coverage for VendorsService and PurchaseOrdersService - approving.