fix(python): import union cases for reflection

[dbrattli]

Summary

• resolve Python reflection union case references through the shared import-aware helper
• import all Result and Choice case classes needed by reflection metadata
• handle System.Text.Rune reflection metadata as a non-generic BCL type
• add regressions for reflected Result record fields and Choice type information

Testing

• ./build.sh test python --skip-fable-library-core
• dotnet build src/Fable.Transforms/Fable.Transforms.fsproj --no-restore -v:minimal

Fixes #5025

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded