[ty] Make the default settings try harder to avoid crashes

[Gankra]

This introduces a safe_mode to our settings type that tells our initialization code to more aggressively believe Settings Failure Is Not An Error. This allows the full settings initialization logic to be used as much as possible for the default database while trying to keep any discarded results as narrow as possible for graceful degradation.

Finally, Python has SFINAE.

Fixes astral-sh/ty#859

[github-actions]

Diagnostic diff on typing conformance tests

No changes detected when running ty on typing conformance tests ✅

[github-actions]

mypy_primer results

No ecosystem changes detected ✅
No memory usage changes detected ✅

[Gankra]

Ah, rude. The exact case of VIRTUAL_ENV=gibberish actually blows up in asserts in dynamic_resolution_paths. This is a big ol' game of whackamole.

[Gankra]

The failure with a gibberish VIRTUAL_ENV is a fascinating maybe-independent bug in our handling of symlinked python environments (homebrew).

We actually handle the gibberish VIRTUAL_ENV value gracefully but then we fallback to grabbing a reference to some homebrew site-packages and then blowup because symlinks are happening and the same dir looks like two different dirs.

(just on main, not with my patches, although my patches don't affect this):

No root found for path '/opt/homebrew/Cellar/python@3.13/3.13.5/lib/python3.13/site-packages'. Known roots: FileRoots(
...
path: "/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages",

Here we have a symlink:

/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/site-packages 
-> 
../../../../../../lib/python3.13/site-packages

[MichaReiser]

You mentioned over chat if we should return a Result with the errors instead of a safe flag. I'm starting to see the appeal of this, if we try to get as much right as possible even in safe mode

The resolution methods could return a result where the error type wraps the fallback settings and the first (or all errors). A caller could then decide to build the program with the fallback settings (or error).

I'm not sure how much harder that is

[Gankra]

We no longer have an input that crashes the default server so I'm going to close this as WONTFIX until someone reports a new one.