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

The Python representation of FormattableString used a field literally
named `str`, producing an object expression with a `@property def str`.
While this runs correctly (the `-> str` return annotation resolves to
the builtin under PEP 563, and Pyright accepts it), Astral's `ty` type
checker resolves the annotation against the class-scoped `str` property
and reports a false-positive `invalid-type-form` error.

Rename the internal field to `fmt`, which sidesteps the builtin-name
collision entirely. The field is internal to the `formattableString`
replacement (written by `Create`, read by `get_Format`), so the change
is fully self-contained.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/Fable.Transforms/Python/Replacements.fs

@@ -1678,8 +1678,8 @@ let formattableString
     (args: Expr list)
     =
     match i.CompiledName, thisArg, args with
-    | "Create", None, [ str; args ] -> objExpr [ "str", str; "args", args ] |> Some
-    | "get_Format", Some x, _ -> getFieldWith r t x "str" |> Some
+    | "Create", None, [ str; args ] -> objExpr [ "fmt", str; "args", args ] |> Some
+    | "get_Format", Some x, _ -> getFieldWith r t x "fmt" |> Some
     | "get_ArgumentCount", Some x, _ ->
         // Use int32(len()) to ensure consistent return type
         let lenExpr =