Fix: Enforce Voucher Validation and Data Transfer Limits in Circuit Relay v2

[seetadev]

@guha-rahul : Thank you so much Rahul for enabling voucher verification and data transfer limit enforcement in a relay protocol.

Wish if you should add test coverage for:

Voucher Validation Tests

Suggested file:
tests/relay/circuit_v2/test_voucher_validation.py

Tests to include:

• test_valid_voucher_is_accepted()
  Simulate a valid signed voucher and assert it's accepted.

• test_invalid_voucher_is_rejected()
  Use a tampered or unsigned voucher and assert rejection.

• test_expired_voucher_is_rejected()
  Create a voucher with a past expiration time and verify it's rejected.

• test_voucher_from_untrusted_peer_is_rejected()
  Test with a voucher signed by an unauthorized peer or entity.

Data Transfer Limit Enforcement Tests

Suggested file:
tests/relay/circuit_v2/test_data_transfer_limits.py

Tests to include:

• test_transfer_within_limit_is_allowed()
  Send data below the cap and ensure the connection is maintained.

• test_transfer_exceeding_limit_is_terminated()
  Exceed limit and assert that the relay drops or throttles the connection.

• test_limit_enforced_per_reservation()
  Ensure per-peer or per-reservation limits are honored across multiple sessions.

Integration-Level Test (Optional but Useful)

Suggested file:
tests/relay/circuit_v2/test_relay_integration.py

Tests to include:

• test_relay_enforces_voucher_and_transfer_limits_combined()
  Simulate a full connection using a voucher, attempt over-transfer, and verify both validation and enforcement.