Standardize token handling with unified token adapter pattern

[Smartdevs17]

Context

Token handling is inconsistent across contract modules.

Current Limitation

Each module handles tokens differently.

Expected Outcome

• Unified token adapter interface
• Consistent token transfer patterns
• Token validation utilities
• Token metadata caching

Acceptance Criteria

• Unified token adapter interface
• Consistent transfer patterns
• Token validation utilities
• Token metadata caching
• Token error handling
• Token testing utilities
• Token documentation

Technical Scope

• stellar-lend/contracts/common/src/ - Common token utils
• stellar-lend/contracts/hello-world/src/ - Token handling
• stellar-lend/contracts/lending/src/ - Token handling
• New crate: stellar-lend/contracts/token/