[Bug]: ConversationService: stored fields mutated in memory before save_meta() with no rollback on failure

ConversationService has a recurring pattern where fields on StoredConversation are mutated in memory before the corresponding save_meta() call persists them to meta.json. If save_meta() fails (disk full, ownership lost, etc.), the in-memory state is ahead of disk. After eviction or restart, the change silently disappears.

This currently affects at least three places:

• update_conversation — mutates stored.title, stored.tags, stored.updated_at
• set_conversation_tag / delete_conversation_tag (via _flush_tags_mutation) — mutates stored.tags
• AutoTitleSubscriber — mutates stored.title, stored.updated_at

Actual Behavior

When save_meta() raises after an in-memory mutation, the change is visible in-process but not persisted. After eviction or restart the mutation disappears silently. The following pytest demonstrates the gap:

# pytest tests/agent_server/test_conversation_tags.py::test_set_tag_rolls_back_on_save_failure
from unittest.mock import patch
with patch.object(event_service, "save_meta", side_effect=RuntimeError("disk full")):
    with pytest.raises(RuntimeError):
        await service.set_conversation_tag(conv_id, "key", "value")
assert "key" not in event_service.stored.tags  # currently fails — key is present despite failed persist

Run with:

pytest tests/agent_server/test_conversation_tags.py

Acceptance Criteria

• set_conversation_tag leaves stored.tags unchanged when save_meta() raises
• delete_conversation_tag leaves stored.tags unchanged when save_meta() raises
• update_conversation leaves stored.title / stored.tags unchanged when save_meta() raises
• AutoTitleSubscriber leaves stored.title unchanged when save_meta() raises

The cleanest fix is reversing the order — write to disk first, then update in-memory state — rather than adding rollback logic after the fact. Not a blocker, but worth addressing consistently across all affected sites rather than piecemeal.

Inspect ConversationService methods update_conversation, set_conversation_tag, delete_conversation_tag, and _flush_tags_mutation, then follow the AutoTitleSubscriber path. Start with tests/agent_server/test_conversation_tags.py and run its pytest suite, adding failure cases for save_meta() errors. Done means each listed mutation leaves in-memory fields unchanged when persistence fails.