fix: validate contract artifact schema in CLI deploy/call path (A-836)

[PhilWindle]

Fixes A-836 (Audit #188).

Problem

The CLI loaded contract artifacts via loadContractArtifact (getContractArtifact in cli/src/utils/aztec.ts). For an already-processed artifact, loadContractArtifact only runs the shallow isContractArtifact heuristic — its own JSDoc notes "The check is not exhaustive". It checks name/functions/nonDispatchPublicFunctions shape but never validates parameter ABIs, types, storage layout, outputs, etc. (Raw nargo output is already fully validated, since it flows through generateContractArtifact → ContractArtifactSchema.parse.)

A malformed-but-superficially-shaped artifact therefore bypassed schema validation and surfaced as an opaque error later during deployment/arg-encoding rather than a clear validation failure.

Fix

Add loadContractArtifactWithValidation in stdlib: it runs the full ContractArtifactSchema over an already-processed artifact before returning, and otherwise defers to loadContractArtifact (raw nargo stays validated as before). The returned object is identical to loadContractArtifact's — the schema parse is used purely as a validation gate. The CLI's getContractArtifact now uses it.

Scoped deliberately to the CLI (the audit's concern). loadContractArtifact is called at 100+ module-load sites with wire-form JSON; adding a stricter schema gate to all of them would have a large blast radius and risk rejecting legacy artifacts, so the unguarded function is left unchanged.

Test

Added loadContractArtifactWithValidation tests in contract_artifact.test.ts: a valid already-processed artifact (wire form) loads, and one whose functionType is a non-enum string — which still passes the shallow isContractArtifact check — is rejected. The rejection test fails without the schema gate and passes with it.

Note

Severity is MEDIUM in the finding, but as the artifact is local input the deployer supplies (not an untrusted network boundary) and downstream code still throws on bad input, the practical impact is closer to LOW (a clearer error message, earlier).