fix: prevent duplicate agent passport registration

[brentthomas248]

Closes #12

Summary:

• Add an AlreadyRegistered contract error for duplicate agent passport registration.
• Reject an existing DataKey::Passport(agent_id) before proof verification can overwrite the stored attestation.
• Preserve the existing passport when a second registration attempts to reuse the same agentId.
• Add focused regression coverage for the duplicate-agent guard and keep the mixed batch InvalidProof case distinct from duplicate registration.

Tests:

• cd contracts/agent-passport-validator && cargo test

[brentthomas248]

Update after assignment: I pushed a format-only follow-up and re-ran verification on the current PR head.

Checks run locally from contracts/agent-passport-validator:

• cargo fmt --check
• cargo test (18 passed)
• cargo clippy --all-targets (completed; warnings are the existing repo-wide Soroban event/test-style warnings)

This remains scoped to #12 and the PR body references Closes #12.