fix(ruby-client): fix incorrect Date parsing in OneOf

Changes Date.parse to Date.iso8601 to fix incorrect type coercion in OneOf contexts. Previously, arbitrary strings matching Date.parse's lenient pattern (e.g., "ABC1") were incorrectly coerced to dates (2025-06-01), when they should have remained as strings.

For example:
- "ABC1" was parsed as Date(2025-06-01) instead of remaining "ABC1"
- This occurred because Date.parse treats the first char as month ('A'=1)
  and remaining digits as day

The fix ensures only ISO8601 formatted strings (e.g. "2025-06-02") are parsed as dates, improving type safety and preventing unexpected coercion of string values.

Author: LuukvH

modules/openapi-generator/src/main/resources/ruby-client/partial_anyof_module.mustache

@@ -56,7 +56,7 @@
         when 'Time'
           return Time.parse(data)
         when 'Date'
-          return Date.parse(data)
+          return Date.iso8601(data)
         when 'String'
           return data if data.instance_of?(String)
         when 'Object' # "type: object"

modules/openapi-generator/src/main/resources/ruby-client/partial_oneof_module.mustache

@@ -98,7 +98,7 @@
         when 'Time'
           return Time.parse(data)
         when 'Date'
-          return Date.parse(data)
+          return Date.iso8601(data)
         when 'String'
           return data if data.instance_of?(String)
         when 'Object' # "type: object"

samples/client/petstore/ruby-httpx/lib/petstore/models/mammal_anyof.rb

@@ -64,7 +64,7 @@ def find_and_cast_into_type(klass, data)
         when 'Time'
           return Time.parse(data)
         when 'Date'
-          return Date.parse(data)
+          return Date.iso8601(data)
         when 'String'
           return data if data.instance_of?(String)
         when 'Object' # "type: object"

samples/client/petstore/ruby-httpx/lib/petstore/models/mammal_without_discriminator.rb

@@ -65,7 +65,7 @@ def find_and_cast_into_type(klass, data)
         when 'Time'
           return Time.parse(data)
         when 'Date'
-          return Date.parse(data)
+          return Date.iso8601(data)
         when 'String'
           return data if data.instance_of?(String)
         when 'Object' # "type: object"

samples/client/petstore/ruby/lib/petstore/models/mammal_anyof.rb

@@ -64,7 +64,7 @@ def find_and_cast_into_type(klass, data)
         when 'Time'
           return Time.parse(data)
         when 'Date'
-          return Date.parse(data)
+          return Date.iso8601(data)
         when 'String'
           return data if data.instance_of?(String)
         when 'Object' # "type: object"

samples/client/petstore/ruby/lib/petstore/models/mammal_without_discriminator.rb

@@ -65,7 +65,7 @@ def find_and_cast_into_type(klass, data)
         when 'Time'
           return Time.parse(data)
         when 'Date'
-          return Date.parse(data)
+          return Date.iso8601(data)
         when 'String'
           return data if data.instance_of?(String)
         when 'Object' # "type: object"