fix: complete contract test suite with all error variants covered

[Mac-5]

closes #6

• Fix Error enum: use #[contracterror] + Copy for proper try_* client methods
• Fix symbol_short! strings to <= 9 chars
• Fix token fee transfer: use TokenClient::transfer (StellarAssetClient has no transfer)
• Redesign create_token: accept pre-deployed token_address, return registry index
• Add transfer_admin function (was missing, referenced in tests)
• Add Debug + PartialEq to TokenInfo for assert_eq! support
• Rewrite test.rs: 21 tests covering all public functions and all Error variants
  • test_initialize, test_initialize_already_initialized
  • test_create_token, test_create_token_insufficient_fee
  • test_set_metadata, test_set_metadata_insufficient_fee
  • test_mint_tokens, test_burn, test_burn_invalid_amount
  • test_update_fees, test_update_fees_unauthorized
  • test_get_token_info, test_get_token_info_not_found
  • pause/unpause, transfer_admin, get_tokens_by_creator tests

Description

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Related Issue

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Type of Change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Testing Done

Please describe the tests you ran (unit tests, manual verification, etc.) to verify your changes.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes

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