withFieldGroup field path mapping for object mappings creates wrong paths

[MGRatEJOT]

Describe the bug

When we use withFieldGroup and a value of the group/form is an object

const MyGroup = withFieldGroup({
  defaultValues: {
    complexValue: {
      prop1: 0,
      prop2: 0
    }
  },
...
})

and then we use this group somewhere like this, assuming form contains a value "complexValue"

<MyGroup 
  form={form}
  fields={{
    complexValue: "complexValue",
  }}
/>

then the internally created mapping by

form/packages/form-core/src/FieldGroupApi.ts

Line 143 in a6abf35

getFormFieldName = <TField extends DeepKeys<TFieldGroupData>>(

is wrong, because the combination of the following code line, which keeps a leading dot from the accessor path
e.g. "complexValue.prop1" results in ".prop1" here.

form/packages/form-core/src/FieldGroupApi.ts

Line 156 in a6abf35

const restOfPath = subfield.slice(firstAccessor.length)

and then later this line

form/packages/form-core/src/utils.ts

Line 199 in a6abf35

return `${path1}.${path2}`

result in a path with two dots in it.

E.g. "complexValue.prop1" ist getting mapped to "complexValue..prop1"

Your minimal, reproducible example

I have none

Steps to reproduce

I hope my description of the bug and the root cause should be enough information to easily track it down and/or reproduce it.

Expected behavior

Correct mappings should get generated by getFormFieldName.

How often does this bug happen?

Every time

Screenshots or Videos

No response

Platform

• OS: Windows
• Browser: Edge
• Version: 138.0.3351.95

TanStack Form adapter

None

TanStack Form version

v1.15.0

TypeScript version

No response

Additional context

No response

[LeCarbonator]

Can you install #1639 and see if that fixes it? @MGRatEJOT

npm i https://pkg.pr.new/@tanstack/react-form@1639