Add __eq__ method to FrozenDict class

[Copilot]

FrozenDict implements __hash__ but not __eq__, violating Python's hashable object contract.

Changes

• Add __eq__ method to FrozenDict that compares underlying root dicts
• Supports comparison with both FrozenDict instances and regular dicts

fd1 = FrozenDict({"a": 1})
fd2 = FrozenDict({"a": 1})
assert fd1 == fd2  # Now works correctly
assert fd1 == {"a": 1}  # Also works

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