fix(ruby-client): fix incorrect Date parsing in OneOf (#21364)

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"