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>

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")