fix(python): avoid union case field name collision with Union.name

[dbrattli]

Problem

Fixes #4645.

A named union case field called name fails at runtime in Python:

type Union =
    | Case of name: string * optional: string

let header1 = Union.Case("v1", "v2")

raises:

TypeError: non-default argument 'optional' follows default argument 'name'

Root cause

The generated case class looked like:

@tagged_union(0)
class Union_Case(_Union):
    name: str
    optional: str

The Union base class (fable_library/union.py) defines a name property. When @dataclass processes the name annotation, it finds that inherited property object via getattr(cls, 'name') and treats it as the field's default value. The next field without a default (optional) then violates Python's "non-default argument follows default argument" rule.

This is the same field-name collision class that records already solve with toFieldSnakeCase (formerly toRecordFieldSnakeCase); union case field generation was the one place still using plain toSnakeCase.

Fix

Use toFieldSnakeCase for union case field annotations, so name becomes name_ and no longer collides:

@tagged_union(0)
class Union_Case(_Union):
    name_: str
    optional: str

This is safe because union construction is positional (Union_Case("v1", "v2")) and field access is by index (self.fields[i]), so the dataclass field name is purely internal.

Rename

Since the toRecordFieldSnakeCase helper is now used for union case fields too — not just records — the "Record" qualifier was misleading. Renamed it to toFieldSnakeCase across the Python transforms and tidied the related comments. (shouldUseRecordFieldNaming stays as-is: it is genuinely record-specific.)

Verification

• Reproduced the original TypeError, confirmed the fix runs correctly.
• Added a regression test in tests/Python/TestUnionType.fs.
• Full Python suite: 2351 passed, no regressions.

🤖 Generated with Claude Code

[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