Fix type safety in Robot Framework JSON validation methods

[Copilot]

json_should_be_valid() was annotated to return dict but returned whatever json.loads() yielded. When the model returned JSON arrays or primitives, json_should_have_keys() raised AttributeError instead of a clear assertion failure.

Changes

• Add isinstance(data, dict) check in json_should_be_valid() after parsing JSON
• Raise AssertionError with clear message when JSON is not an object: "Expected JSON object (dict), but got list"
• Document assumption in json_should_have_keys() that upstream validates dict type

Example

# Before: AttributeError: 'list' object has no attribute 'keys'
lib._last_response = '[1, 2, 3]'
lib.json_should_have_keys("name", "age")  # Crashes

# After: AssertionError: Expected JSON object (dict), but got list.
# Value: [1, 2, 3]

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