fix date string format validation (#130)

Quoting from the JSON schema validation vocabulary

> date:  A string instance is valid against this attribute if it is a
>        valid representation according to the "full-date" production.

and from RFC 3339

```
date-fullyear   = 4DIGIT
date-month      = 2DIGIT  ; 01-12
date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on
                            ; month/year
full-date       = date-fullyear "-" date-month "-" date-mday
```

The current implementations allows date strings that don't comply, for
example `"12-12-12"`. The regex based check fixes that.

References:
* OAS 3.0.0 https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.0.md#dataTypes
* JSON validation vocabulary https://datatracker.ietf.org/doc/html/draft-bhutton-json-schema-validation-00#section-7.3
* RFC 3339, section 5.6 https://datatracker.ietf.org/doc/html/rfc3339#section-5.6

Co-authored-by: Nemo Oudeis <dfrey@moneytree.jp>

Author: NemoOudeis

lib/openapi_parser/schema_validator/string_validator.rb

@@ -71,6 +71,8 @@ def validate_uuid_format(value, schema)
       def validate_date_format(value, schema)
         return [value, nil] unless schema.format == 'date'
 
+        return [nil, OpenAPIParser::InvalidDateFormat.new(value, schema.object_reference)] unless value =~ /^\d{4}-\d{2}-\d{2}$/
+
         begin
           parsed_date = Date.iso8601(value)
         rescue ArgumentError

spec/openapi_parser/schema_validator/string_validator_spec.rb

@@ -236,7 +236,29 @@
         it do
           expect { subject }.to raise_error do |e|
             expect(e).to be_kind_of(OpenAPIParser::InvalidDateFormat)
-            expect(e.message).to end_with("Value: \"not_date\" is not conformant with date format")
+            expect(e.message).to end_with("Value: \"#{value}\" is not conformant with date format")
+          end
+        end
+      end
+      context 'incomplete date string (2 digit year)' do
+        let(:value) { '12-12-12' }
+        let(:params) { { 'date_str' => value } }
+
+        it do
+          expect { subject }.to raise_error do |e|
+            expect(e).to be_kind_of(OpenAPIParser::InvalidDateFormat)
+            expect(e.message).to end_with("Value: \"#{value}\" is not conformant with date format")
+          end
+        end
+      end
+      context 'invalid date string format (/ separator)' do
+        let(:value) { '12/12/12' }
+        let(:params) { { 'date_str' => value } }
+
+        it do
+          expect { subject }.to raise_error do |e|
+            expect(e).to be_kind_of(OpenAPIParser::InvalidDateFormat)
+            expect(e.message).to end_with("Value: \"#{value}\" is not conformant with date format")
           end
         end
       end