fix(admissions): flag-off pages must not speak for the council

The secondary admissions section words the absence of a cut-off distance:
"<LA> has not published a cut-off distance for this school." That sentence
is true when the authority publishes nothing. It is false when the
authority does publish and the admission_distance flag is simply off — and
off is the current state, so every secondary page with an EES admissions
row has been making a claim about a council on our behalf.

The backend already draws the distinction the copy needs. /api/schools/{urn}
omits the admission_distance key entirely while the flag is dark rather than
sending null, precisely so that "we are not publishing cut-offs" stays
distinguishable from "this school has no cut-off"; lib/types.ts says so in
as many words. The page then collapsed the two with `?? null` before the
section ever saw them.

So stop collapsing it: thread the raw field to SecondarySchoolSections and
word the absence only when the feature is on. Null still gets the sentence
naming the authority — that case is unchanged and still tested.

Primary pages are unaffected: AdmissionsSection carries no absence copy and
renders nothing when there is no figure. DistanceSection already treated
absent and null alike; only its type widens.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FuPUioHpxtaiDNagQvjxyM

nextjs-app/__tests__/components/lastDistanceOffered.test.tsx

@@ -98,6 +98,17 @@ describe('secondary detail page', () => {
 
     expect(screen.getByText(/has not published a cut-off distance/)).toBeInTheDocument();
   });
+
+  it('makes no claim about publication when the feature is switched off', () => {
+    // Absent, not null. The API omits the key entirely while the
+    // admission_distance flag is off, and "Islington has not published a
+    // cut-off distance" is then a statement about us, not about Islington —
+    // false wherever the authority does publish one.
+    renderSecondarySchoolDetail({ ...secondaryFixture, admissionDistance: undefined });
+
+    expect(screen.queryByText(/has not published a cut-off distance/)).not.toBeInTheDocument();
+    expect(screen.queryByText(/Contact the admissions authority/)).not.toBeInTheDocument();
+  });
 });
 
 // ── The Distance section ───────────────────────────────────────────────