Edit session silently decouples from active source when pipeline node is switched mid-session

[pcolt]

Description

When a user starts an edit session and then clicks a different node in the pipeline browser, the session is not aborted or warned — but the backend's active source silently switches to the new node while EditSession.working_dataset still holds a deep copy of the original node's mesh.

Steps to reproduce

1. Load a mesh and start an edit session (Edit Tools → Begin Edit Session).
2. While the session is active, click a different node in the pipeline browser.
3. The viewport updates to show the new node, but edit_session_active remains True and the edit panel stays open.
4. Make a selection and commit.

What goes wrong

• on_active_pipeline_item_change (state_handlers.py:56) has no guard for edit_session_active, so pv_backend.set_active_node() runs unconditionally.
• After the switch, pv_backend.source points to the new node, but edit_session.working_dataset is still the deep copy of the original node.
• Any pick/selection event is now dispatched against the wrong source in the backend.
• A commit saves the session's working dataset (from the original node) to the original filename, regardless of what is displayed.

Risk

Silent data-integrity issue: the user may believe they are editing the currently displayed mesh, but edits land on the original node's file.

Candidate fixes

• Block switching (simplest): in on_active_pipeline_item_change, return early if state.edit_session_active is True and optionally show a notification explaining why the switch was ignored.
• Prompt to discard (friendlier): detect the switch attempt, show a confirmation dialog ("Discard current edit session and switch node?"), and only proceed if confirmed.