fix(history): defer full transcript hydration to model sends (#5929)

* fix(history): defer full hydration to model sends

* fix(session): key hydration on real rows, fork through get_session

Two regressions from the display/model-context split, both reproducible
against dev.

The hydration gate compared the cached transcript against the
denormalized sessions.message_count column. That column drifts in normal
operation — _persist_message swallows a failed insert while add_message
has already appended in memory, so the next successful persist writes
rows+1 — and _db_to_session re-read the same column after each reload, so
the shortfall never closed. Every send, edit, delete and truncate on a
warm session re-selected the whole message table: the cost this change
set out to remove, relocated onto the hot path. The other direction was
just as bad — a persist for an uncached session writes message_count = 0,
and a stale-low counter with a partly filled cache meant no hydration at
all and a silently truncated transcript for the model.

sync_session_metadata now reconciles message_count against COUNT(*) on
chat_messages (one indexed count inside the connection it already opens),
and _db_to_session trusts the rows it just loaded. A hydrate always
closes the gap, so the next read is a cache hit.

fork_session read session_manager.sessions directly and never hydrated.
keep_count indexes into source.history, and display pagination no longer
fills that cache, so forking after a restart returned HTTP 200 with an
empty conversation and no error surfaced. It goes through get_session
now.

_hydrate_session_history_from_db is gone with its helper: get_session is
the hydration seam, and rebuilding session.history from raw rows in the
display fallback overwrote the parsed multimodal content and the _db_id
edit/delete keys that had just been set.

Tests drive a real SessionManager over a temp DB instead of a stub that
only proved the stub hydrates — both drift directions, the send path
warm and cold, and a fork taken after a restart. All five fail without
this change. The brittle SQL-text assertions are dropped; the page
bounds are already proven by the response body.

* fix(history): route pagination through canonical handler

---------

Co-authored-by: Léo <leograndcontact@gmail.com>

tests/test_fork_session_metadata.py

@@ -34,6 +34,11 @@ class _FakeSessionManager:
         self.sessions = {"src-id": source}
         self.created = None
 
+    def get_session(self, session_id):
+        # Fork looks the source up through get_session — the hydration seam —
+        # so a session only present in the DB still forks a real transcript.
+        return self.sessions[session_id]
+
     def create_session(self, session_id=None, name=None, endpoint_url=None,
                        model=None, rag=False, owner=None):
         self.created = _FakeSession(name=name, owner=owner)