Getting proc instead of string message

## Describe the bug

I am getting a callable proc in the following code.

## To Reproduce

```ruby
require 'dry-validation'

class TestContract < Dry::Validation::Contract
params do
required(:last_name).filled(:string)
optional(:birthday).maybe(:date)
end

rule(:birthday) do
key.failure('wrong date')
end

rule(:last_name) do
result.errors.to_h
end
end

validator = TestContract.new
puts validator.call(last_name: 'Last', birthday: '5.01.0202').errors.to_h

# {:birthday=>[#]}

```

## Expected behavior

`# {:birthday=>["wrong date"]}`

## My environment

- Affects my production application: **NO**
- Ruby version: 3.3.0
- OS: macOS 13.6.7

# Workaround

Make a copy of errors:

`result.errors.dup.to_h`