fix(backend): stop showing the clinician null, NaN and the wrong failure

Two ways a voice failure described itself wrongly.

describe() built the quoted-back text from fields that are all nullable on
the wire, and toVoiceIntent casts rather than checks — so a half-classified
deadline rendered as “null null” — not a usable date, and an offset with no
amount as “+NaN day”. Blank is already handled by the sheet; it now falls
back to that.

The DTO's constraints resolved to unrelated codes: maxLength fell through
to VALIDATION_FIELD_REQUIRED, so an oversized recording said a field was
missing, and isIn maps to VALIDATION_LANGUAGE_INVALID, so an unsupported
container said the language was invalid. Both now name their own code —
the validation factory already returns a message verbatim when it is itself
a known ErrorCode, so this needs no change to the shared mapping.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/modules/voice/due-date.resolver.spec.ts

@@ -346,3 +346,40 @@ describe('resolveDueDate', () => {
     });
   });
 });
+
+describe('what an unresolvable deadline quotes back', () => {
+  // The wire shape allows nulls in every field and toVoiceIntent casts rather than
+  // checks, so these reach the resolver intact. The sheet renders `spoken` verbatim.
+  it('never puts "null" or "NaN" in front of the clinician', () => {
+    const bad = [
+      { kind: 'weekday', weekday: null, which: null },
+      { kind: 'offset', unit: null, amount: null },
+      { kind: 'offset', unit: 'day', amount: Number.NaN },
+      { kind: 'jalali', jy: null, jm: 7, jd: 25 },
+      { kind: 'gregorian', y: 2026, m: null, d: null },
+    ];
+    for (const intent of bad) {
+      const result = resolveDueDate(
+        intent as unknown as DueIntent,
+        SATURDAY,
+        FA_WEEK,
+      );
+      expect(result.dueDate).toBeNull();
+      expect(result.unresolved?.spoken ?? '').not.toMatch(/null|NaN/);
+    }
+  });
+
+  it('still quotes a deadline it did understand the words of', () => {
+    const result = resolveDueDate(
+      {
+        kind: 'weekday',
+        weekday: 'thursday',
+        which: null,
+      } as unknown as DueIntent,
+      SATURDAY,
+      FA_WEEK,
+    );
+    // A weekday with no "this/next" resolves, so nothing is quoted back at all.
+    expect(result.dueDate).not.toBeNull();
+  });
+});