I dunno what exactly I was thinking but we definitely don't want to
**ever** raise from the original exc-group, instead always raise from
any original `.__cause__` to be consistent with the embedded src-error's
context.
Also, adjust `maybe_collapse_eg()` to return `False` in the non-single
`.exceptions` case, again don't know what I was trying to do but this
simplifies caller logic and the prior return-semantic had no real
value..
This fixes some final usage in the runtime (namely top level nursery
usage in `._root`/`._runtime`) which was previously causing test suite
failures prior to this fix.
Since it's for beg filtering, the current impl should be renamed anyway;
it's not just for filtering cancelled excs.
Deats,
- added a real doc string, links to official eg docs and fixed the
return typing.
- adjust all internal imports to match.
Inside a new `.trionics._beg` and exposed from the subpkg ns in
anticipation of the `strict_exception_groups=False` being removed by
`trio` in py 3.15.
Notes,
- mk an embedded single-exc "extractor" using a `BaseExceptionGroup.exceptions` length
check, when 1 return the lone child.
- use the above in a new `@acm`, async bc it's most likely to be composed in an
`async with` tuple-style sequence block, called `collapse_eg()` which
acts a one line "absorber" for when the above mentioned flag is no
logner supported by `trio.open_nursery()`.
All untested atm fwiw.. but soon to be used in our test suite(s) likely!
Tyler Goodlet