Support contextual session management

[elronbandel]

Adds Python contextvars support to enable implicit session usage,
providing a cleaner API while maintaining full backwards compatibility.

Changes

• New context manager: session() that manages session lifecycle
• Implicit in-session functions: instruct(), chat(), validate(), query(),
transform() use current active context session (and fail if called outside the context)
• Updated generative slots: @Generative functions can now work without explicit
  session parameter
• Backwards compatible: All existing code continues to work unchanged

Usage

New (cleaner):

from mellea import session, chat, generative

@generative
def add_one(x: int) -> int:
    ...

with session("ollama", "granite"):
     response = chat("Tell me more")
     two = add_one(1)

Still works:

  m = start_session("ollama", "granite")
  result = m.chat("Tell me more")

Also works:

 with session("ollama", "granite") as m:
      result = m.chat("Tell me more")

[nrfulton]

I do worry a bit about confusion arising from session vs start_session vs MelleaSession but I'm probably just being paranoid.

[elronbandel]

It is confusing. For now I added the context management ability to start_session so we have only start_session which now works also with with start_session():. In the future we can consider having one name that represents both modes, and deprecating this one. Anyways if and when it should be separate PR.

[nrfulton]

I'm inclined to merge; I like it. Adding @HendrikStrobelt as a reviewer to make sure I'm not missing anything obvious.

@elronbandel did you test this with mobject stuff?

[elronbandel]

@nrfulton I added comprehensive tests, including mified objects.

[nrfulton]

LGTM. We need to create some documentation about this, probably in the Context Management section of the tutorial I'll create a new issue for this and merge for now, so that we're not gating a new feature on docs.