fix: show hover in notebook when kernal switches

[NarxPal]

Summary

Cell content metdata was being updated when there are changes made to cells inside the notebook. LspEvent's didChange event runs the notebook_document_did_change fn which earlier updated the cell metdata of notebook only when there are changes made to the cell and since kernel switching doesn't cause any cell changes, the existing cells didn't get inserted into notebookDocument struct's cell field. this caused hover to stop working on the existing cells after kernel switching.

Fixes #3598

Test Plan

• Manually verified in VS Code using a local Pyrefly build from this branch.
• created a test.ipynb file, created new cell in it.
• wrote x=5 , and switched kernel and hovered over x

[kinto0]

thanks!

is there any way to test this behavior with a lsp_interaction test that failed before your changes?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[NarxPal]

previously test_notebook_hover_basic fn was not handling the hover over cell_content after kernel change, so i added that step in another fn called test_notebook_hover_after_kernel_change and since kernel change causes an lsp didChange event we use the change_notebook fn to stimulate the behaviour of kernel change.

[meta-codesync]

@kinto0 has imported this pull request. If you are a Meta employee, you can view this in D107533803.

[kinto0]

thank you! clean fix with great test coverage