doc updates, localize test fixtures

doc updates, localize test fixtures

testing with types is inherently awkward and subject
to changes in python interpreters (such as all the recent python 3.14
stuff we had them fix), but in this suite we already have a lot of
types that are defined inline inside of test methods.   so since that's
how many of the tests work anyway, organize the big series of pep-695
and pep-593 structures into fixtures or individual tests to make
the whole suite easier to follow.   pyright complains quite a lot
about this, so if this becomes a bigger issue for say mypy /pep484
target, we may have to revisit (which I'd likely do with more ignores)
or if function/method-local type declarations with global becomes a runtime
issue in py3.15 or something, we can revisit then where we would in
theory need to convert the entire suite, which I'd do with a more
consistent naming style for everything.

but for now try to go with fixtures / local type declarations so that
we dont have to wonder where all these types are used.

For 2.0 this further repairs some merge mismatches between 2.1 and
2.0, with tests lining up more closely to where they were placed in
2.1.

Change-Id: Ibe8f447eaa10f5e927b1122c8b608f11a5f5bc97
(cherry picked from commit 8f7138326cf48a1394417aa492ad083b3400c529)