Fixes #14762

There was a confusing state you could get into where when you navigate off of a positron notebook into a non-editor-type context like a data explorer etc we never triggered the event that caused the assistant chat to think it had left the notebook. This meant that the notebook was still "active" in the chat pane (e.g. the active runtime was the notebook) but when the assistant went to query the notebook the thing we gate on is the notebook being the active editor which if focus is on something like the data explorer then it is not. The result was a confused agent and user because the agent didnt have access to the notebook context it expected. This fixes that by letting the notebook tools search open but not focused editors for the correct notebook.

Agent generated notes 👇

positron.notebooks.getContext resolved the notebook only from the active editor pane, so assistant tools reported "no notebook is open in the editor" whenever focus was anywhere else (a Data Explorer or plot editor opened from the notebook, the chat editor, a split group, another tab) even though a Positron notebook was open. This PR adds a fallback: when no Positron notebook is the active pane, getContext resolves the open notebook attached to the foreground session (what the interpreter picker shows, and what assistant notebook mode is keyed on), then the most recently active open Positron notebook.

Two edge behaviors are deliberate:

• When the active editor is a notebook in the built-in (Jupyter) editor, getContext still rejects with the actionable "reopen in the Positron Notebook Editor" error rather than falling back to a different open notebook.
• When the foreground session's notebook editor has been closed, resolution falls back to the most recently active open notebook rather than targeting a closed one. (Assistant tool calls that pass an explicit notebook path verify it against the resolved notebook and error on mismatch, so a stale session can't silently redirect a path-addressed edit.)

Release Notes

New Features

• N/A

Bug Fixes

• Fix Assistant failing to find an open Positron Notebook when the notebook is not the active editor pane, e.g. after opening a Data Explorer or moving focus to another editor (Assistant has trouble finding Positron Notebooks even when they're open. #14762)

Validation Steps

@:positron-notebooks @:assistant

Regression tests: src/vs/workbench/api/test/browser/positron/mainThreadNotebookFeatures.vitest.ts covers the fallback (open-but-inactive notebook, MRU order across multiple notebooks, foreground-session preference, closed foreground notebook, nothing open, and the built-in-editor error).

Manual check:

1. Open a Positron Notebook (positron.notebook.enabled on) and run a cell so it has a session.
2. Open a Data Explorer from the notebook (e.g. View a dataframe) so the notebook is no longer the active editor pane.
3. Ask Posit Assistant something that reads the notebook (or invoke a notebook tool). It should find the notebook instead of replying "no notebook is open in the editor".
4. With a second notebook also open, verify the assistant still targets the notebook attached to the current session, not the more recently touched one.
5. Open a notebook in the built-in Jupyter editor as the active tab; notebook tools should still return the "reopen in the Positron Notebook Editor" guidance.