fix(gateway): preserve transcript on /compress and hygiene compression (salvage #3516) (#3556)

* fix(gateway): preserve full transcript on /compress instead of overwriting

The /compress command calls _compress_context() which correctly ends the
old session (preserving its full transcript in SQLite) and creates a new
session_id for the continuation. However, it then immediately called
rewrite_transcript() on the OLD session_id, overwriting the preserved
transcript with the compressed version — destroying searchable history.

Auto-compression (triggered by context pressure) does not have this bug
because the gateway already handles the session_id swap via the
agent.session_id != session_id check after _run_agent_sync.

Fix: after _compress_context creates the new session, write the compressed
messages into the NEW session_id and update the session store pointer.
The old session's full transcript stays intact and searchable via
session_search.

Before: /compress destroys original messages, session_search can't find
details from compressed portions.

After: /compress behaves like /new for history — full transcript preserved,
compressed context for the live session.

* fix(gateway): preserve transcript on /compress and hygiene compression

Apply session_id swap after _compress_context in both /compress handler
and hygiene pre-compression. _compress_context creates a new session
(ending the old one), but both paths were calling rewrite_transcript on
the OLD session_id — overwriting the preserved transcript and destroying
searchable history.

Now follows the same pattern as the auto-compression handler (lines
5415-5423): detect the new session_id, update the session store entry,
and write compressed messages to the new session.

Also fix FakeCompressAgent test mock to include session_id attribute
and simulate the session_id change that real _compress_context performs.

Co-authored-by: MacroAnarchy <MacroAnarchy@users.noreply.github.com>

---------

Co-authored-by: MacroAnarchy <MacroAnarchy@users.noreply.github.com>

gateway/run.py

@@ -2204,6 +2204,15 @@ class GatewayRunner:
                                     ),
                                 )
 
+                                # _compress_context ends the old session and creates
+                                # a new session_id.  Write compressed messages into
+                                # the NEW session so the old transcript stays intact
+                                # and searchable via session_search.
+                                _hyg_new_sid = _hyg_agent.session_id
+                                if _hyg_new_sid != session_entry.session_id:
+                                    session_entry.session_id = _hyg_new_sid
+                                    self.session_store._save()
+
                                 self.session_store.rewrite_transcript(
                                     session_entry.session_id, _compressed
                                 )
@@ -3998,13 +4007,22 @@ class GatewayRunner:
             loop = asyncio.get_event_loop()
             compressed, _ = await loop.run_in_executor(
                 None,
-                lambda: tmp_agent._compress_context(msgs, "", approx_tokens=approx_tokens),
+                lambda: tmp_agent._compress_context(msgs, "", approx_tokens=approx_tokens)
             )
 
-            self.session_store.rewrite_transcript(session_entry.session_id, compressed)
+            # _compress_context already calls end_session() on the old session
+            # (preserving its full transcript in SQLite) and creates a new
+            # session_id for the continuation.  Write the compressed messages
+            # into the NEW session so the original history stays searchable.
+            new_session_id = tmp_agent.session_id
+            if new_session_id != session_entry.session_id:
+                session_entry.session_id = new_session_id
+                self.session_store._save()
+
+            self.session_store.rewrite_transcript(new_session_id, compressed)
             # Reset stored token count — transcript changed, old value is stale
             self.session_store.update_session(
-                session_entry.session_key, last_prompt_tokens=0,
+                session_entry.session_key, last_prompt_tokens=0
             )
             new_count = len(compressed)
             new_tokens = estimate_messages_tokens_rough(compressed)

tests/gateway/test_session_hygiene.py

@@ -304,8 +304,12 @@ async def test_session_hygiene_messages_stay_in_originating_topic(monkeypatch, t
     class FakeCompressAgent:
         def __init__(self, **kwargs):
             self.model = kwargs.get("model")
+            self.session_id = kwargs.get("session_id", "fake-session")
+            self._print_fn = None
 
         def _compress_context(self, messages, *_args, **_kwargs):
+            # Simulate real _compress_context: create a new session_id
+            self.session_id = f"{self.session_id}_compressed"
             return ([{"role": "assistant", "content": "compressed"}], None)
 
     fake_run_agent = types.ModuleType("run_agent")