fleetd #689: pin the answerGatePasses call site via the audit trail

A unit test on the extracted helper proves the helper, not the call
site in sendMessage. allow() logs an AuditLog.allowed() entry for
every granted non-READ/METRICS/TASK_READ action, so a granted turnId
request must log both SEND and ANSWER, and a granted plain request
must log SEND alone. Verified this goes red when the call site is
deleted from sendMessage, and restores to a clean diff.

fleetd/src/test/java/dev/ltms/fleet/rest/FleetAppAuthTest.java

@@ -1,5 +1,8 @@
 package dev.ltms.fleet.rest;
 
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import com.fasterxml.jackson.databind.ObjectMapper;
 import dev.ltms.fleet.auth.CallerResolver;
 import dev.ltms.fleet.auth.Authz;
 import dev.ltms.fleet.auth.MemberRegistry;
@@ -18,6 +21,7 @@ import dev.ltms.fleet.msg.Rendezvous;
 import dev.ltms.fleet.session.FakeWorktrees;
 import dev.ltms.fleet.session.SessionManager;
 import dev.ltms.fleet.member.ClaudeCodeLauncher;
+import dev.ltms.fleet.testing.CapturedLog;
 import io.javalin.Javalin;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
@@ -292,6 +296,58 @@ class FleetAppAuthTest {
         assertTrue(granted.body().contains("stale_turn"), "a granted caller's body IS read and acted on");
     }
 
+    /**
+     * fleetd #689 (ticket comment 18353): the only place {@code sendMessage}'s call to {@code
+     * answerGatePasses} is observable is the audit trail — {@code allow()} logs an {@code
+     * "allowed"} entry for every granted action except {@code READ}/{@code METRICS}/{@code
+     * TASK_READ}, and {@code ANSWER} is none of those. A granted {@code turnId} request must
+     * therefore log both a {@code SEND} and an {@code ANSWER} entry; a granted plain request must
+     * log {@code SEND} alone. A unit test of the extracted helper pins the helper; this pins the
+     * call site — deleting the {@code answerGatePasses} call from {@code sendMessage} leaves the
+     * helper's own test green but turns this one red.
+     */
+    @Test
+    void aGrantedTurnIdRequestAuditsBothSendAndAnswerButAPlainRequestAuditsSendAlone() throws Exception {
+        int port = start(999_999, false, null); // primary: granted both SEND and ANSWER
+        ObjectMapper mapper = new ObjectMapper();
+
+        try (CapturedLog audit = CapturedLog.at("audit", Level.INFO)) {
+            send(port, "POST", "/sessions/term_b/message",
+                    "{\"turnId\":\"turn-1\",\"content\":\"hi\"}", null);
+
+            List<String> allowed = allowedActions(audit, mapper);
+            assertTrue(allowed.contains("SEND"),
+                    "a turnId request must still clear the coarse SEND grant first");
+            assertTrue(allowed.contains("ANSWER"),
+                    "a turnId request must ALSO clear the ANSWER grant — this is the call site itself");
+        }
+
+        try (CapturedLog audit = CapturedLog.at("audit", Level.INFO)) {
+            send(port, "POST", "/sessions/term_b/message",
+                    "{\"content\":\"hi\",\"timeoutMs\":50}", null);
+
+            List<String> allowed = allowedActions(audit, mapper);
+            assertEquals(List.of("SEND"), allowed,
+                    "a plain request must log SEND and nothing else — ANSWER is conditional on "
+                            + "turnId, not something every request happens to log");
+        }
+    }
+
+    private static List<String> allowedActions(CapturedLog audit, ObjectMapper mapper) {
+        return audit.events().stream()
+                .map(ILoggingEvent::getFormattedMessage)
+                .map(line -> {
+                    try {
+                        return mapper.readTree(line);
+                    } catch (Exception e) {
+                        throw new AssertionError("audit line is not valid JSON: " + line, e);
+                    }
+                })
+                .filter(n -> "allowed".equals(n.path("outcome").asText()))
+                .map(n -> n.path("action").asText())
+                .toList();
+    }
+
     // --- token mode ---------------------------------------------------------------------------
 
     @Test