[Python] Redesign Discriminated Union (DU) type

[dbrattli]

Background

For Python we would like a Discriminated Union (DU) type that

1. Preserves type safety - Python type checkers (pyright, mypy) should understand the union structure
2. Enables pattern matching - Python 3.10+ match statements should work naturally with case extraction
3. Provides good IDE support - Autocomplete, go-to-definition, and hover documentation should work
4. Maintains backwards compatibility - Existing Fable Python code using .tag and .fields should continue to work
5. Follows Python conventions - The generated code should look and feel like idiomatic Python

This PR changes the generated Union code:

F# Source

type MyUnion =
    | CaseA of int
    | CaseB of string
    | CaseC of x: float * y: float

Generated Python Output

from fable_library.union import Union, tagged_union

# Base class with underscore prefix (private/internal)
class _MyUnion(Union):
    """Base class inheriting from Union for compatibility."""

    @staticmethod
    def cases() -> list[str]:
        return ["CaseA", "CaseB", "CaseC"]


@tagged_union(0)
class MyUnion_CaseA(_MyUnion):
    item: int


@tagged_union(1)
class MyUnion_CaseB(_MyUnion):
    item: str


@tagged_union(2)
class MyUnion_CaseC(_MyUnion):
    x: float
    y: float


# Type alias - THE public union type for annotations
type MyUnion = (MyUnion_CaseA | MyUnion_CaseB) | MyUnion_CaseC

Closes: #3558

[github-actions]

Python Type Checking Results (Pyright)

❌ Type errors found in non-excluded files

Metric	Value
Excluded files	3

Files excluded from type checking

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

temp/tests/Python/fable_modules/thoth_json_python/encode.py
temp/tests/Python/test_applicative.py
temp/tests/Python/test_misc.py