#164: a backend-error failure must carry the whole scrape, not just the matched line

The BACKEND_ERROR pattern is a heuristic. It also matches a member that forgot
fleet_reply while reporting *about* a backend error. Failing is still right --
the caller must never read a scrape as an answer -- but the reason carried only
the matched line, so the rest of the report was thrown away.

That is the same defect #164 exists to fix: information destroyed on the way to
the caller. Carry the full pane tail alongside the classification, so a genuine
backend error reads the same as before and a false positive keeps its report.

fleetd/src/main/java/dev/ltms/fleet/inject/CompletionResolver.java

@@ -292,7 +292,12 @@ public final class CompletionResolver implements TurnListener {
         // scrape that reads like a completed answer.
         String backendError = firstMatchingLine(assistantBlock, BACKEND_ERROR);
         if (backendError != null) {
-            fail(target, turn, "member " + target + " ended on a backend error: " + backendError);
+            // Carry the whole scrape, not just the matched line. The pattern is a heuristic: a member
+            // that forgot fleet_reply while reporting *about* a backend error matches it too. Failing
+            // is still right — the caller must not read a scrape as an answer — but dropping the rest
+            // of the pane would destroy the report, which is the same defect fleetd#164 is about.
+            fail(target, turn, "member " + target + " ended on a backend error: " + backendError
+                    + "\n--- pane tail ---\n" + tail);
             return;
         }
         String completion = clipped ? tail + "\n" + CLIPPED_PANE_TAIL_MARKER : tail;

fleetd/src/test/java/dev/ltms/fleet/inject/CompletionResolverTest.java

@@ -612,6 +612,27 @@ class CompletionResolverTest {
         assertEquals(Rendezvous.Kind.FAILED, waiter.getNow(null).kind(), "the pattern is case-insensitive");
     }
 
+    @Test
+    void aBackendErrorFailureStillCarriesTheRestOfTheScrape() {
+        // The pattern is a heuristic: a member that forgot fleet_reply while *reporting on* a backend
+        // error matches it too. Failing is still correct, but the report itself must survive — losing
+        // it would be the same information-destroying defect fleetd#164 exists to fix.
+        String block = "\u23fa I looked into the gateway problem.\n"
+                + "The log line was: API Error: 400 invalid request body\n"
+                + "The cause is a missing content-type header.\n\u276f ";
+        FakeHerdr herdr = new FakeHerdr().readText(block);
+        Rendezvous rendezvous = new Rendezvous();
+        CompletionResolver resolver = new CompletionResolver(new AgentControl(herdr), rendezvous, ExhaustedPatternLookup.none(), ExhaustionSink.none());
+
+        var waiter = rendezvous.open("term_a");
+        resolver.resolve("term_a", new CompletionResolver.InFlight(waiter, null));
+
+        String reason = waiter.getNow(null).text();
+        assertEquals(Rendezvous.Kind.FAILED, waiter.getNow(null).kind());
+        assertTrue(reason.contains("The cause is a missing content-type header."),
+                "the failure carries the rest of the pane, not only the matched line: " + reason);
+    }
+
     @Test
     void coverageIsOffWhenNoProfileHasAPatternConfigured() {
         assertEquals("off (no profile has an exhaustedPattern configured; profiles: [terra])",