fix(eims): map buyer Wereda to a MoR code too, fail locally if unmapped

BuyerDetails.Wereda had the same problem Region did: companies.woreda holds
names ("Yeka") MoR has no confirmed regex for, but every Wereda value MoR has
actually shown us (seller "12"/"13", the collection's "574") is 1-3 digits
like Region. Precautionary, not confirmed -- but the fix is identical either
way: resolve through EIMS_BUYER_WEREDA_CODES and refuse to file rather than
send a guessed code.

Generalises the Region resolver (resolveRegionCode -> resolveLocationCode) to
cover both fields instead of duplicating it.

No code was invented for "Yeka" -- EIMS_BUYER_WEREDA_CODES ships empty, so
this buyer now fails locally (new stop) instead of silently sending a name
that was never verified against MoR's schema.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.spec.ts

@@ -61,6 +61,7 @@ const context = (over: Partial<EimsMapperContext> = {}): EimsMapperContext => ({
   incomeWithholdValue: 0,
   transactionWithholdValue: 0,
   buyerRegionCodes: { "Addis Ababa": "13" },
+  buyerWeredaCodes: {},
   ...over,
 });
 
@@ -230,7 +231,7 @@ describe("toEimsInvoice — MoR field constraints", () => {
         seller,
         context(),
       ),
-    ).toThrow(/not a MoR region code and has no mapping/);
+    ).toThrow(/not a MoR Region code and has no mapping/);
   });
 
   it("refuses a buyer with no region at all rather than guessing one", () => {
@@ -240,7 +241,31 @@ describe("toEimsInvoice — MoR field constraints", () => {
         seller,
         context(),
       ),
-    ).toThrow(/buyer region \(unset\)/);
+    ).toThrow(/buyer Region \(unset\)/);
+  });
+
+  it("passes a buyer wereda through when it is already a MoR code", () => {
+    const doc = toEimsInvoice(invoice(), seller, context());
+    expect(doc.BuyerDetails.Wereda).toBe("574");
+  });
+
+  it("maps a wereda name to its code", () => {
+    const doc = toEimsInvoice(
+      invoice({ company: { ...invoice().company!, woreda: "Yeka" } }),
+      seller,
+      context({ buyerWeredaCodes: { Yeka: "99" } }),
+    );
+    expect(doc.BuyerDetails.Wereda).toBe("99");
+  });
+
+  it("refuses to file a buyer whose wereda has no mapping", () => {
+    expect(() =>
+      toEimsInvoice(
+        invoice({ company: { ...invoice().company!, woreda: "Yeka" } }),
+        seller,
+        context({ buyerWeredaCodes: {} }),
+      ),
+    ).toThrow(/buyer Wereda "Yeka".*EIMS_BUYER_WEREDA_CODES/);
   });
 
   it("emits NatureOfSupplies lowercase, whatever case it was configured in", () => {