Validate token contract exists before group creation

[WuXieSec]

This PR adds validation to ensure the provided token address is a valid Stellar asset contract, as requested in #62.

Changes:

• Call token contract's symbol() to verify it exists
• Return InvalidToken error if contract call fails
• Add InvalidToken variant to ContractError enum
• Add test coverage for valid and invalid tokens

Why this matters:
Without validation:

• Groups could be created with invalid token addresses
• Contributions would fail later with cryptic errors
• Users would waste gas on broken groups
• Contract state would be polluted with unusable groups

Implementation:
Uses try_symbol() to safely check if the token contract exists and implements the token interface. If the call fails, group creation is rejected with InvalidToken.

Test coverage:

• ✅ Valid token contract → group created successfully
• ❌ Random address (not a contract) → InvalidToken
• ❌ Non-token contract address → InvalidToken

Closes #62