lambda's parameter is marked as untyped with union

[pocke]

Lambda's parameter is marked as untyped unexpectedly when the lambda is assigned to a variable as a union type.

# It is ok. `str` is `String`
# @type var x: ^(String) -> void
x = -> (str) { str + 'a' }

# It is not ok. `str` is `untyped`, but I expected `String`.
# @type var x: ^(String) -> void | nil
x = -> (str) { str + 'a' }

I've confirmed this problem with vscode and Steep LSP's hover feature.

---

Actually I found this problem when I tried to enable checking "untyped call" on retryable gem's RBS.
https://github.com/ruby/gem_rbs_collection/blob/006177988c5ad5261efe67af814cc0f726b0eb8e/gems/retryable/3.0/configuration.rbs#L23
https://github.com/ruby/gem_rbs_collection/blob/006177988c5ad5261efe67af814cc0f726b0eb8e/gems/retryable/3.0/_test/test.rb#L68
In this case, retries is marked as untyped unexpectedly.