Adapt to plumpy's greenlet async bridge

[khsrali]

This accompanies the plumpy PR that replaces nest_asyncio with greenlet-based bridging.
AiiDA has several places where synchronous code needs to call async operations while the event loop is running — process execution, the kill path, and async transport operations. Each needs adapting to work within the greenlet model.

Additionally, dropping nest_asyncio breaks Jupyter notebooks --there's absolutely no way not to break it!--:
The kernel has a running event loop, there's no greenlet context.

The only backward incompatible change, In notebooks, users write:

result = await multiply.run_async(x, y)
result = await engine.run_async(builder)

Since Jupyter supports top-level await, this works naturally. The async path calls run_in_greenlet internally, so all the nested execution machinery works correctly. That means no need for asyncio.run(), etc from the user side.
We solve this by providing an async API in line with sync ones. I believe this is a more decent solution over the patching or working around the event loop, as it was before.

Anyways, in scripts (where the loop is NOT running), the synchronous API still works as before:

result = multiply(x, y)
result = engine.run(builder)

[codecov]

Codecov Report

❌ Patch coverage is 34.53237% with 91 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.48%. Comparing base (be364ad) to head (74984a2).
⚠️ Report is 122 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aiida/engine/processes/functions.py	16.67%	45 Missing ⚠️
src/aiida/engine/runners.py	15.79%	32 Missing ⚠️
src/aiida/engine/launch.py	33.34%	12 Missing ⚠️
src/aiida/transports/transport.py	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7188      +/-   ##
==========================================
- Coverage   79.66%   79.48%   -0.17%     
==========================================
  Files         565      565              
  Lines       43637    43769     +132     
==========================================
+ Hits        34759    34786      +27     
- Misses       8878     8983     +105     

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[khsrali]

I think the tests are failing because if yet another nested loop in transport:

src/aiida/transports/transport.py:180: in __enter__
    self.open()
src/aiida/transports/transport.py:1838: in open
    return self.run_command_blocking(self.open_async)
src/aiida/transports/transport.py:1835: in run_command_blocking
    return loop.run_until_complete(func(*args, **kwargs))
src/aiida/engine/runners.py:160: in run_until_complete
    return self._loop.run_until_complete(future)

I hope we can hand this over to plumpy, otherwise we may need to greenlet it inside aiida-core
to be investigated...

EDIT:
Problem solved

[khsrali]

Ok, it works now! 🎊

[khsrali]

closed in favour of #7206