Renderer's `._session` should be made public

[schloerke]

When looking at great-tables, they make use of the meta data session for session._process_ui().

Given our (@wch) conversations on making sure the session being used within the Renderer is the Session from init time only (and not runtime, possibly within a module).

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Implementation thoughts:

• Move ._session to .session

• Before calling

  py-shiny/shiny/session/_session.py

  Line 1411 in 4c28e91

  value = await renderer.render()

  , we could set the session context to (Output) self.session

Option 1 ( making the variable public ) seems like a simpler approach as users will not need ask for the session every time they need access