refactor(python): rename FormattableString field from str to fmt

[dbrattli]

Problem

The Python representation of FormattableString used a field literally named str, so a $"...{x}..." interpolation typed as FormattableString compiled to an object expression with a @property def str:

class ObjectExpr2405:
    @property
    def str(self, __unit: Unit=UNIT) -> str:   # <- field named `str`
        return "You owe: {0:N5} {1} {2}"
    @property
    def args(self, __unit: Unit=UNIT) -> Array[Any]:
        return Array[Any]([int32(100), int32.THREE, True])

This runs correctly and Pyright accepts it — under PEP 563 (from __future__ import annotations) the -> str return annotation resolves to the builtin (get_type_hints confirms {'return': <class 'str'>}), and the property never shadows the builtin since attribute names live in a separate namespace.

However, Astral's ty resolves the annotation against the class-scoped str property and reports a false positive:

error[invalid-type-form]: Variable of type `property` is not allowed in a return type annotation
    --> temp/tests/Python/test_string.py:1039:45
1039 |         def str(self, __unit: Unit=UNIT) -> str:

Fix

Rename the internal field str → fmt in the Python formattableString replacement. The field is internal to that one function (written by Create, read by get_Format), so the change is fully self-contained — GetArgument/GetArguments/ArgumentCount use the separate args field, and GetStrings() is JS-only.

Verification

• All 2350 Python tests pass (./build.sh test python)
• ty check temp/tests/Python/test_string.py → All checks passed!
• Pyright remains clean

🤖 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