Validate metadata_uri is non-empty in credential-nft mint

[DeFiVC]

What

The credential-nft contract stores metadata_uri as-is with no validation that it's a well-formed URI or non-empty. A credential could be minted with an empty or meaningless URI.

Why

Credentials with empty or invalid metadata URIs are useless to holders and can't be verified off-chain. Adding a basic validation prevents accidental misconfiguration and ensures every minted credential has meaningful metadata.

Scope

• Add a minimum length check on metadata_uri (e.g., non-empty, minimum 8 characters)
• Consider basic URI format validation (starts with http://, https://, or ipfs://)
• Add a test verifying rejection of empty/invalid URIs

Technical Context

• File: contracts/credential-nft/src/mint.rs (line ~107)
• metadata_uri is passed as a SorobanString and stored directly

Acceptance Criteria

• mint_credential panics when metadata_uri is empty
• mint_credential panics when metadata_uri is too short or malformed
• Valid URIs are accepted without change
• Unit tests verify both rejection and acceptance