refactor(python): split FSharpRef.__init__ into overloads

[dbrattli]

Summary

The FSharpRef stub declared a single constructor with a union parameter:

def __init__(self, contents_or_getter: T | Callable[[], T],
             setter: Callable[[T], None] | None = None) -> None: ...

This splits it into two overloads matching the two real construction modes — wrap a value, or wrap a getter/setter pair:

@overload
def __init__(self, contents_or_getter: T) -> None: ...
@overload
def __init__(self, contents_or_getter: Callable[[], T], setter: Callable[[T], None]) -> None: ...

Motivation

Pyright infers T from the union form fine, but ty cannot solve T through T | Callable[[], T] for the getter/setter form — it binds T to the getter callable itself, yielding FSharpRef[T] that it then rejects at every out-parameter call site (try_parse, byref helpers, etc.). The overloads make T unambiguously solvable from each mode. Runtime behaviour is identical and the intent is clearer.

This came out of an investigation into whether ty could replace Pyright for this repo; the single-constructor union was the largest single source of ty false positives.

Verification

	ty (tests)	ty (library)	Pyright (tests)	Pyright (library)	runtime
before	292	72	34	0	2350 ✓
after	118	68	34	0	2350 ✓

• Removes 178 false-positive ty diagnostics across the generated output.
• No change to Pyright (the gating checker): still 0 library / 34 tests.
• All 2350 Python tests pass (./build.sh test python).

🤖 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