fix(gateway): carry origin_json/display_name into /branch child sessions too

Complete the /branch routing-identity fix (salvaged from PR #62278 by
@jcjc81): in addition to user_id/session_key/chat_id/chat_type/thread_id,
forward origin_json and display_name at create_session() time, matching
the reset-path db_create_kwargs pattern (#82633) so the branch row is
born with full identity — no backfill gap for state.db consumers
(mcp_serve, mirror, channel directory) if a crash lands before
switch_session().

The obsolete compression-rotation half of #62278 was dropped: rotation
now goes exclusively through publish_compression_child, which already
copies all identity columns in-transaction.

gateway/slash_commands.py

@@ -4774,6 +4774,20 @@ class GatewaySlashCommandsMixin:
 
         parent_session_id = current_entry.session_id
 
+        # Serialize the parent's full origin (same shape as the reset path's
+        # db_create_kwargs in gateway/session.py, #82633) so the branch row
+        # carries complete identity from birth. Prefer the live entry's origin
+        # (it may hold richer metadata than the triggering event's source).
+        _branch_origin = current_entry.origin or source
+        _branch_origin_json = None
+        if _branch_origin is not None:
+            try:
+                import json as _json
+
+                _branch_origin_json = _json.dumps(_branch_origin.to_dict())
+            except Exception:
+                _branch_origin_json = None
+
         # Create the new session with parent link.
         # Persist a stable ``_branched_from`` marker in model_config so
         # list_sessions_rich() keeps the branch visible in /resume and
@@ -4798,12 +4812,19 @@ class GatewaySlashCommandsMixin:
                 # IDOR guard too (which requires the row's chat_id/thread_id to
                 # match the caller's). user_id is critical for the fallback lookup
                 # path (hermes_state.py:1994-2009) that searches by the complete
-                # peer tuple when session_key doesn't match.
+                # peer tuple when session_key doesn't match. origin_json and
+                # display_name complete the identity (same shape as the reset
+                # path's db_create_kwargs in gateway/session.py, #82633) so
+                # consumers that read routing/presentation data from state.db
+                # (mcp_serve, mirror, channel directory) see the branch row
+                # fully formed with zero backfill gap.
                 user_id=source.user_id,
                 session_key=session_key,
                 chat_id=source.chat_id,
                 chat_type=source.chat_type,
                 thread_id=source.thread_id,
+                origin_json=_branch_origin_json,
+                display_name=current_entry.display_name,
             )
         except Exception as e:
             logger.error("Failed to create branch session: %s", e)

tests/gateway/test_branch_routing_columns.py

@@ -137,6 +137,19 @@ class TestBranchRoutingColumns:
         assert row["session_key"] is not None, (
             "branched session lost session_key — primary lookup path fails"
         )
+        # origin_json completes the identity (#82633 reset-path pattern):
+        # consumers reading routing/presentation data from state.db
+        # (mcp_serve, mirror, channel directory) need the full origin on
+        # the branch row without waiting for any backfill.
+        assert row["origin_json"], (
+            "branched session lost origin_json — state.db consumers see an "
+            "identity-less branch row until a peer refresh backfills it"
+        )
+        import json as _json
+
+        origin = _json.loads(row["origin_json"])
+        assert origin.get("chat_id") == "170829464"
+        assert origin.get("thread_id") == "544520"
 
         _ = real_switch_session  # silence unused