feat(memory): notify providers on mid-process session_id rotation (#17409)

Fixes #6672

Memory providers now receive on_session_switch() whenever AIAgent.session_id
rotates mid-process — /resume, /branch, /reset, /new, and context
compression. Before this, providers that cached per-session state in
initialize() (Hindsight's _session_id, _document_id, accumulated
_session_turns, _turn_counter) kept writing into the old session's
record after the agent had moved on.

MemoryProvider ABC
------------------
- New optional hook on_session_switch(new_session_id, *,
  parent_session_id='', reset=False, **kwargs) with no-op default for
  backward compat. reset=True signals /reset or /new — providers should
  flush accumulated per-session buffers. reset=False for /resume,
  /branch, compression where the logical conversation continues.

MemoryManager
-------------
- on_session_switch() fans the hook out to every registered provider.
  Isolated try/except per provider — one bad provider can't block others.
- Empty/None new_session_id is a no-op to avoid corrupting provider state
  during shutdown paths.

run_agent.py
------------
- _sync_external_memory_for_turn now passes session_id=self.session_id
  into sync_all() and queue_prefetch_all(). Providers with defensive
  session_id updates in sync_turn (Hindsight already had this at
  plugins/memory/hindsight/__init__.py:1199) now actually receive the
  current id.
- Compression block at ~L8884 already notified the context engine of
  the rollover; now also calls
  _memory_manager.on_session_switch(reason='compression').

cli.py
------
- new_session() fires reset=True, reason='new_session' so providers
  flush buffers.
- _handle_resume_command fires reset=False, reason='resume' with the
  previous session as parent_session_id.
- _handle_branch_command fires reset=False, reason='branch' with the
  parent session_id already captured for the DB parent link.

gateway/run.py
--------------
- _handle_resume_command now evicts the cached AIAgent, mirroring
  /branch and /reset. The next message rebuilds a fresh agent whose
  memory provider initialize() runs with the correct session_id —
  matches the pattern the gateway already uses for provider state
  cross-session transitions.

Hindsight reference implementation
----------------------------------
- plugins/memory/hindsight/__init__.py adds on_session_switch that:
  updates _session_id, mints a fresh _document_id (prevents
  vectorize-io/hindsight#1303 overwrite), and clears _session_turns /
  _turn_counter / _turn_index so in-flight batches don't flush under
  the new document id. parent_session_id only overwritten when provided
  (avoids clobbering on a bare switch).

Tests
-----
- tests/agent/test_memory_session_switch.py: new dedicated file. ABC
  default no-op, manager fan-out, failure isolation, empty-id no-op,
  session_id propagation through sync_all/queue_prefetch_all, Hindsight
  state transitions for every reset/non-reset case, parent preservation.
- tests/cli/test_branch_command.py: new test verifying /branch fires
  the hook with correct parent_session_id + reset=False + reason.
- tests/gateway/test_resume_command.py: new test verifying /resume
  evicts the cached agent.
- tests/run_agent/test_memory_sync_interrupted.py: updated existing
  assertions to account for the session_id kwarg on sync_all and
  queue_prefetch_all.

E2E verified (real imports, tmp HERMES_HOME):
- /resume: session_id updates, doc_id fresh, buffers cleared, parent set
- /branch: session_id forks, parent links to original
- /new: reset=True clears accumulated state
- compression: reason='compression' propagated, lineage preserved
- Empty id: no-op, state preserved
- Legacy provider without on_session_switch: no crash

Reported by @nicoloboschi (Hindsight maintainer); related scope-widening
comment by @kidonng extending coverage to compression.

tests/cli/test_branch_command.py

@@ -192,6 +192,33 @@ class TestBranchCommandCLI:
 
         assert cli_instance._resumed is True
 
+    def test_branch_fires_on_session_switch_hook(self, cli_instance, session_db):
+        """The /branch command must notify memory providers of the rotation.
+
+        Without this, providers that cache per-session state in
+        initialize() keep writing under the old session_id. See #6672.
+        """
+        from cli import HermesCLI
+
+        # Wire a real-ish agent object with a MagicMock memory_manager
+        agent = MagicMock()
+        mm = MagicMock()
+        agent._memory_manager = mm
+        cli_instance.agent = agent
+        original_id = cli_instance.session_id
+
+        HermesCLI._handle_branch_command(cli_instance, "/branch")
+
+        # Hook must have been called exactly once with the new session_id,
+        # parent pointing at the branched-from session, reset=False, and
+        # reason="branch" for diagnostics.
+        assert mm.on_session_switch.call_count == 1
+        _, kwargs = mm.on_session_switch.call_args
+        assert mm.on_session_switch.call_args.args[0] == cli_instance.session_id
+        assert kwargs["parent_session_id"] == original_id
+        assert kwargs["reset"] is False
+        assert kwargs["reason"] == "branch"
+
     def test_fork_alias(self):
         """The /fork alias should resolve to 'branch'."""
         from hermes_cli.commands import resolve_command