Rename to SessionSnapshot, move unwind assertion closer

[dhruvmanila]

This PR addresses the post-merge review comments from #19041, specifically it:

• Rename WorkspaceSnapshot to SessionSnapshot
• Rename take_workspace_snapshot to take_session_snapshot
• Rename take_snapshot to take_document_snapshot
• Move AssertUnwindSafe closer to the catch_unwind call which requires the assertion

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

parso (https://github.com/davidhalter/parso)
-     memo fields = ~49MB
+     memo fields = ~54MB

bidict (https://github.com/jab/bidict)
-     memo fields = ~17MB
+     memo fields = ~15MB

beartype (https://github.com/beartype/beartype)
- TOTAL MEMORY USAGE: ~97MB
+ TOTAL MEMORY USAGE: ~88MB

ignite (https://github.com/pytorch/ignite)
-     memo fields = ~189MB
+     memo fields = ~171MB

jinja (https://github.com/pallets/jinja)
-     memo fields = ~88MB
+     memo fields = ~80MB

pytest (https://github.com/pytest-dev/pytest)
- TOTAL MEMORY USAGE: ~276MB
+ TOTAL MEMORY USAGE: ~251MB

isort (https://github.com/pycqa/isort)
- TOTAL MEMORY USAGE: ~88MB
+ TOTAL MEMORY USAGE: ~97MB

altair (https://github.com/vega/altair)
-     memo fields = ~251MB
+     memo fields = ~228MB

bokeh (https://github.com/bokeh/bokeh)
- TOTAL MEMORY USAGE: ~251MB
+ TOTAL MEMORY USAGE: ~276MB

openlibrary (https://github.com/internetarchive/openlibrary)
-     memo fields = ~189MB
+     memo fields = ~171MB

manticore (https://github.com/trailofbits/manticore)
-     memo fields = ~652MB
+     memo fields = ~593MB

[MichaReiser]

Thank you