Optimize Storage Keys in Soroban Smart Contracts

[Fatimasanusi]

The current fractional token contracts use long String-based storage keys, which increases gas costs for read/write operations on the Stellar network. We should optimize these to use Enums or shorter Symbol types.

Acceptance Criteria:

• Refactor contract storage keys from String to symbol_short! or enum DataKey where applicable.
• Update the contract SDK and tests to reflect the new storage key structures.
• Run gas profiling to measure the reduction in storage fees.