response forward compatibility test

[sbansla]

Added unit test for response forward compatibility testing.

Test Scenarios Covered:
MessageInstance (flat response)

• Ignores an unknown top-level field
• Deserializes a removed field to nil
• Preserves an unknown status value
• Deserializes an empty payload to an all-nil resource

ChannelsSenderInstance (nested, top-level response)

• Ignores an unknown field at a nested level
• Removes a field inside a nested object while siblings survive
• Deserializes a removed whole nested object to nil
• Keeps a new item appended to a nested list
• Preserves an unknown status value at a nested level
• Raises for a truly unknown property (NoMethodError), as opposed to an unknown JSON field

RecallInstance (nested, operation-specific response)

• Ignores an unknown field at the parent level and at a nested level
• Deserializes a removed field at the parent level and inside a nested object to nil/absent
• Preserves an unknown enum-like value inside a nested list item