Fix subcommand edge cases associated with generics, tyro.conf.Suppress, tyro.conf.AvoidSubcomands

[brentyi]

Now fixed:

• tyro.conf.AvoidSubcommands could unnecessarily narrow types when generic types were placed in unions.
• x: tyro.conf.Suppress[StructA | None] = None was throwing an error if a default was missing from StructA.

Example of one bug we had:

from dataclasses import dataclass
import tyro

@dataclass(frozen=True)
class Container[T]:
    value: T | bool

def main(
    container: Container[int] | Container[str] = Container("default string"),
) -> None:
    print(container.value)

if __name__ == "__main__":
    tyro.cli(main, config=(tyro.conf.AvoidSubcommands,))

The help that we expect and now get is:

usage: test.py [-h] [--container.value STR|{True,False}]

╭─ options ─────────────────────────────────────────╮
│ -h, --help        show this help message and exit │
╰───────────────────────────────────────────────────╯
╭─ container options ───────────────────────────────╮
│ --container.value STR|{True,False}                │
│                   (default: 'default string')     │
╰───────────────────────────────────────────────────╯

Compared to previously:

/Users/brentyi/miniconda3/envs/py312/lib/python3.12/site-packages/tyro/_resolver.py:480: UserWarning: <class 'str'> does not match any type in Union: [<class 'int'>, <class 'bool'>]
  warnings.warn(
usage: test.py [-h] [--container.value {fixed}]

╭─ options ─────────────────────────────────────────╮
│ -h, --help        show this help message and exit │
╰───────────────────────────────────────────────────╯
╭─ container options ───────────────────────────────╮
│ --container.value {fixed}                         │
│                   (fixed to: default string)      │
╰───────────────────────────────────────────────────╯

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (735530a) to head (4e67115).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #220   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files          28       28           
  Lines        2296     2307   +11     
=======================================
+ Hits         2292     2303   +11     
  Misses          4        4           

Flag	Coverage Δ
unittests	99.82% <100.00%> (+<0.01%)	⬆️

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