fix: add context to verification key size errors

[loglapa]

Fixes #25107.

Summary

• Validate private-function verification key byte lengths while the contract and function names are still available.
• Preserve the expected and actual byte counts and suggest rebuilding stale or incompatible artifacts.
• Add a regression test for the contextual error message.

Testing

• make yarn-project in aztecprotocol/devbox:3.0
• yarn format stdlib
• yarn workspace @aztec/stdlib test src/abi/contract_artifact.test.ts (7 tests passed)
• git diff --check

[loglapa]

Rebased onto the current merge-train/spartan tip. Validation in aztecprotocol/devbox:3.0: immutable install, full TypeScript build, the focused contract_artifact.test.ts suite (7/7), format check, lint, and git diff --check all pass. The repository-wide bootstrap reaches the unchanged ipc-runtime prerequisite and stops because this devbox warns that Rust 1.89.0 is not installed; the yarn-project build and changed tests are green. A maintainer still needs to add ci-external-once (contributors cannot add repository labels).