#302: fleet_reply refuses blank content too, matching fleet_send

MessageService.reply now throws on blank content. FleetMcp.reply guarded only
against null, and its handler is a bare BiFunction with no try/catch, so a
whitespace-only fleet_reply left the handler as an uncaught
IllegalArgumentException instead of the clean tool error null already got.
fleet_send has always used isBlank here; reply now matches it.

The worker found that null/isBlank difference and reported it as a correction
to my ticket, which had quoted the guard wrongly. It was right: I grepped the
error string and assumed the condition matched its sibling.

fleetd/src/main/java/dev/ltms/fleet/mcp/FleetMcp.java

@@ -813,7 +813,12 @@ public final class FleetMcp {
             return error("fleet_reply is for workers only — could not identify the calling worker "
                     + "from the connection");
         }
-        if (content == null) {
+        // fleetd #302: isBlank, not == null, to match fleet_send's own guard above. MessageService
+        // .reply now REJECTS blank content, and this handler is a bare BiFunction with no try/catch
+        // around it — so a whitespace-only fleet_reply would leave here as an uncaught
+        // IllegalArgumentException instead of this clean tool error. Null and whitespace are the
+        // same mistake by the caller and must get the same answer.
+        if (isBlank(content)) {
             return error("content is required");
         }
         messages.reply(callerTerminal, content);

fleetd/src/test/java/dev/ltms/fleet/mcp/FleetMcpTest.java

@@ -326,6 +326,26 @@ class FleetMcpTest {
         assertEquals("orphan", drained.getFirst().content());
     }
 
+    @Test
+    void replyWithBlankContentIsACleanToolErrorNotAnUncaughtException() {
+        // fleetd #302: MessageService.reply now REJECTS blank content by throwing. fleet_reply's
+        // handler is a bare BiFunction with no try/catch around it, so if this guard only checked
+        // `== null` (as it did), a whitespace-only reply would leave the handler as an uncaught
+        // IllegalArgumentException instead of a tool error the caller can read. Null and whitespace
+        // are the same caller mistake and must get the same answer — the sibling fleet_send guard
+        // has always used isBlank for exactly this reason.
+        for (String blank : new String[] {null, "", "   ", "\n\t"}) {
+            McpSchema.CallToolResult res = assertDoesNotThrow(
+                    () -> FleetMcp.reply(messages, "term_a", blank),
+                    "blank content must be refused as a tool error, never thrown out of the handler");
+            assertEquals(Boolean.TRUE, res.isError(), "blank content is an error result");
+            assertTrue(textOf(res).contains("content is required"),
+                    "the error names the missing argument: " + textOf(res));
+        }
+        assertEquals(0, messages.drainReplies("term_a").size(),
+                "a refused reply must not reach the inbox");
+    }
+
     @Test
     void bridgePollWithTargetDrainsReplies() {
         // A reply with no open send queues it in the inbox.