[Contract] Add get_organizer_address helper

[Yunusabdul38]

Description: Centralize the logic for retrieving an organizer's address from an event ID. This helper simplifies cross-contract communication and internal lookups. Tasks:

Implement get_organizer_address(env: Env, event_id: String) -> Result<Address, EventRegistryError>.
Ensure it returns EventNotFound if the ID doesn't exist.
Update existing manual lookups to use this helper.

[Sundriveauto]

@Sundriveauto has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

pls i i will love to work on this

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This issue has been completed by @Sundriveauto as part of the Stellar Wave Program's 3rd Wave 🥳

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