Fixes #271

Problem:
When setIn tries to remove a nested field key from a parent that contains
a string error message (e.g., 'Invalid range' on 'range' when removing
'range.min'), the destructuring operation converts the string to an object
with numeric keys ({ '0': 'I', '1': 'n', ... }).

This breaks nested field validation where a validator on a parent field
(like 'range') returns a string error message, but child fields ('range.min',
'range.max') also exist.

Root Cause:
Line 61 in setIn.ts: `const { [key]: _removed, ...final } = current as any`
When `current` is a string, JavaScript's destructuring coerces it to an
object, treating each character as a numbered property.

Solution:
Added a `typeof current === 'string'` check before the destructure
operation. Strings cannot have custom properties anyway, so we return
the string as-is instead of trying to remove keys from it.

Changes:
- src/structure/setIn.ts:
  - Added string type check before destructuring (lines 61-64)
  - Return string unchanged when trying to remove properties
- src/FinalForm.validating.test.ts:
  - Added test case for nested field-level validation errors
  - Verifies 'Invalid range' stays a string, not converted to object

Impact:
✅ Nested field validators work correctly
✅ String error messages preserved on parent fields
✅ No breaking changes
✅ All existing tests pass

…527)

Fixes #271

Problem:
When setIn tries to remove a nested field key from a parent that contains
a string error message (e.g., 'Invalid range' on 'range' when removing
'range.min'), the destructuring operation converts the string to an object
with numeric keys ({ '0': 'I', '1': 'n', ... }).

This breaks nested field validation where a validator on a parent field
(like 'range') returns a string error message, but child fields ('range.min',
'range.max') also exist.

Root Cause:
Line 61 in setIn.ts: `const { [key]: _removed, ...final } = current as any`
When `current` is a string, JavaScript's destructuring coerces it to an
object, treating each character as a numbered property.

Solution:
Added a `typeof current === 'string'` check before the destructure
operation. Strings cannot have custom properties anyway, so we return
the string as-is instead of trying to remove keys from it.

Changes:
- src/structure/setIn.ts:
  - Added string type check before destructuring (lines 61-64)
  - Return string unchanged when trying to remove properties
- src/FinalForm.validating.test.ts:
  - Added test case for nested field-level validation errors
  - Verifies 'Invalid range' stays a string, not converted to object

Impact:
✅ Nested field validators work correctly
✅ String error messages preserved on parent fields
✅ No breaking changes
✅ All existing tests pass

Co-authored-by: Erik Rasmussen <[email protected]>