Added a duration type fix

[djthorpe]

This pull request improves support for custom types derived from time.Duration that implement their own MarshalJSON method. Previously, such types were incorrectly rendered as integers in generated JSON schemas rather than as duration strings. The changes ensure these types are correctly identified and their schema is set to "string" with format "duration". Several regression tests are added to verify this behavior.

Custom duration type handling:

• Added the isDurationLikeType function to detect named types derived from time.Duration that implement json.Marshaler, ensuring they are treated as duration strings in schemas.
• Updated applySpecialTypeSchema and marshalDefault to use isDurationLikeType, so custom duration types are rendered with type "string" and format "duration". [1] [2]

Testing improvements:

• Introduced the customDuration type and customDurationStruct for testing, simulating a common pattern where packages define local duration types with custom JSON marshaling.
• Added regression tests (TestFor_CustomDuration_TopLevel and TestFor_CustomDuration_StructField) to verify that custom duration types are properly handled in both top-level and struct field contexts.

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