fix(hooks): prevent rejected inbound conversation claims (#109857)

* fix(hooks): honor rejected inbound conversations

* test(hooks): trim rejection fixture

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/hooks/message-hook-mappers.test.ts

@@ -464,6 +464,24 @@ describe("message hook mappers", () => {
     });
   });
 
+  it("does not fall back when a channel rejects inbound claim resolution", () => {
+    const canonical = deriveInboundMessageHookContext(
+      makeInboundCtx({
+        Provider: "claim-chat",
+        Surface: "claim-chat",
+        OriginatingChannel: "claim-chat",
+        From: undefined,
+        To: "channel:room-123",
+        OriginatingTo: "channel:room-123",
+        GroupChannel: undefined,
+        GroupSubject: undefined,
+      }),
+    );
+
+    expect(toPluginInboundClaimContext(canonical).conversationId).toBeUndefined();
+    expect(toPluginInboundClaimEvent(canonical).conversationId).toBeUndefined();
+  });
+
   it("passes thread parent ids to channel plugin claim resolvers", () => {
     const canonical = deriveInboundMessageHookContext(
       makeInboundCtx({

src/hooks/message-hook-mappers.ts

@@ -323,7 +323,11 @@ function resolveInboundConversation(canonical: CanonicalInboundMessageHookContex
         threadParentId: canonical.threadParentId,
         isGroup: canonical.isGroup,
       })
-    : null;
+    : undefined;
+  if (pluginResolved === null) {
+    // A plugin-owned null is an explicit rejection, so generic parsing must not reclaim it.
+    return {};
+  }
   if (pluginResolved) {
     return {
       conversationId: normalizeOptionalString(pluginResolved.conversationId),