Fix invalid quest ID error handling,

[gerani1]

Description

closes #211.

This PR significantly improves the developer experience and contract robustness by removing raw panic! strings and replacing them with a formal QuestChainError enum using Soroban's #[contracterror] attribute. This standardization allows front-end clients and indexers to parse and handle explicit error codes instead of arbitrary panic strings.

Key Changes

• Introduced QuestChainError Enum: Defined a standardized set of explicit error variants (e.g., AlreadyInitialized, InvalidAdmin, InvalidChainId, QuestNotFound) covering all contract validation failures.
• Refactored Contract Panics: Migrated all generic panic!(...) occurrences inside QuestChainContract to use soroban_sdk::panic_with_error!(&env, QuestChainError::Variant).
• Updated Test Suite:
  • Adjusted all #[should_panic] assertions across the quest_chain test suite to expect the new typed format (e.g., expected = "Error(Contract, #N)").
  • Addressed pre-existing logic bugs in tests (like missing initialization arguments and invalid whitespace in Symbol instantiation) to ensure the test suite compiles and runs flawlessly with the new changes.

Verification

• All unit tests in contracts/quest_chain compile and pass.
• Error codes successfully emit via HostError to standard Soroban events payload.
• Unused testing variables were cleaned up.

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