fix: enhance comparison logic for zero-length since

[XuJiandong]

No description provided.

[Copilot]

Pull request overview

This PR updates EpochNumberWithFraction comparison semantics to behave safely and consistently when an epoch is constructed with a zero length (e.g., via unchecked construction or malformed on-chain data), and adds unit tests covering the comparison behavior.

Changes:

• Normalize zero-length epochs during PartialOrd comparison to avoid incorrect ordering when length == 0.
• Adjust cross-multiplication logic to use the normalized (index, length) pair.
• Add unit tests for epoch comparisons, including zero-length edge cases.

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[mohanson]

It appears we checked if length was 0 when constructing EpochNumberWithFraction, but this only works in debug mode.

debug_assert!(number < Self::NUMBER_MAXIMUM_VALUE);
debug_assert!(index < Self::INDEX_MAXIMUM_VALUE);
debug_assert!(length < Self::LENGTH_MAXIMUM_VALUE);
debug_assert!(length > 0);
debug_assert!(index < length);

Perhaps it would be more appropriate to put this logic in the constructor?

[XuJiandong]

The previous(old) implementation is acceptable. from_full_value should handle length = 0, as it may originate from a transaction. It should disallow explicitly setting length = 0.