apollo_staking: implement get_previous_epoch in CairoStakingContract

[dafnamatsry]

Note

Medium Risk
Touches contract-call behavior and low-level retdata deserialization; a mismatch with the contract’s actual ABI/encoding would surface as runtime parsing errors or incorrect epoch handling.

Overview
CairoStakingContract.get_previous_epoch is now implemented to call the on-chain get_previous_epoch_data view and deserialize an optional Epoch result instead of todo!().

To support this, contract_types adds reusable Cairo1 retdata deserialization helpers for Felt, Option<T> and a CairoOption<T> wrapper, refactors Epoch/array parsing to use the iterator-based approach, and expands unit/integration tests to cover Some/None, invalid variants/lengths, and contract-level previous-epoch behavior.

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[reviewable-StarkWare]

This change is 

[dafnamatsry]

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• apollo_staking: reorganize contract_types.rs into TryFromIterator and TryFrom<Retdata> sections #13064
• apollo_staking: implement get_previous_epoch in CairoStakingContract #12902 👈 (View in Graphite)
• apollo_staking: implement get_previous_epoch_data in mock staking contract #12901
• main-v0.14.2

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[matanl-starkware]

@matanl-starkware reviewed 4 files and all commit messages, and made 2 comments.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on dafnamatsry).

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crates/apollo_staking/src/contract_types.rs line 31 at r1 (raw file):

}

impl TryFromIterator<Felt> for Felt {

I think this, and other "generic non staking related" stuff should be moved to a different location.

Code quote:

impl TryFromIterator<Felt> for Felt {

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crates/apollo_staking/src/contract_types_test.rs line 164 at r1 (raw file):

#[case::empty_retdata(vec![])]
#[case::some_wrong_length(vec![Felt::ZERO, Felt::ONE])]
#[case::none_wrong_length(vec![Felt::ONE, Felt::TWO])]

Missing case for "too many args"