feat(token-factory): validate create_token params

[Jayydy]

Description

Please include a summary of the changes and the "Why" behind them.

Summary

Adds input validation to create_token in the TokenFactory contract,
putting the previously dead Error::InvalidParameters variant to use.

Changes

contracts/token-factory/src/lib.rs

• Added a validation block at the top of create_token (before auth,
  fee, and storage access) that rejects:
  • empty name
  • empty symbol
  • decimals > 18
  • initial_supply < 0
• Fixed pre-existing compilation errors: symbol_short! strings
  exceeding the 9-char limit, wrong StellarAssetClient import,
  missing transfer_admin method, invalid deploy_token API call,
  and Error enum missing #[contracterror] / #[repr(u32)].

contracts/token-factory/src/test.rs

• Added 5 unit tests covering each invalid parameter case:
  • test_create_token_rejects_empty_name
  • test_create_token_rejects_empty_symbol
  • test_create_token_rejects_decimals_19
  • test_create_token_accepts_decimals_18
  • test_create_token_rejects_negative_supply

contracts/token-factory/Cargo.toml

• Moved proptest from [dependencies] to [dev-dependencies].

Test results

All 22 unit tests pass (cargo test -p token-factory).

Acceptance criteria

• Empty token name returns InvalidParameters
• Empty symbol returns InvalidParameters
• decimals > 18 returns InvalidParameters
• InvalidParameters error variant is no longer dead code

Related Issue

Link the issue using "Closes #XX" or "Fixes #XX".

Closes #93

Type of Change

Please tick the relevant options:

• [x ] 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

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

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