feat: add get_wrap_hash convenience query

[VictorEzenma]

I have completed the implementation of get_wrap_hash and verified the code correctness using cargo check.

Description

Added a new read-only query method get_wrap_hash to the StellarWrapContract to allow lightweight clients to retrieve only the data hash for a
user-period. This improves efficiency for clients performing integrity checks by avoiding the overhead of loading the full WrapRecord and its
associated fields (timestamp, archetype, etc.).

Changes:

• Added get_wrap_hash to src/queries.rs to query the DataKey::Wrap and return only the data_hash field.
• Exposed get_wrap_hash in the StellarWrapContract implementation within src/lib.rs.
• Added test_get_wrap_hash in src/test.rs to verify the method works correctly for existing wraps and returns None for nonexistent ones.

Closes #279

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