[harrshita123]

Summary

• Allow test vectors to name a key-generation or byte-operation failure and assert its error type and message.
• Validate the inputs for the selected operation so unrelated data is not silently ignored.
• Add AES-GCM vectors for invalid key length and tag length, plus focused runner coverage for signing, verification, encryption, decryption, and key derivation.

This is a follow-up to #55. PR #450 covers the issue's key-import example; this change extends the runner to generation and byte operations. It does not add stream-operation or key-export error vectors.

Testing

• dart test --exclude-tags browser-interop,sanitizer -p vm,chrome -c dart2js,dart2wasm -r compact — 4,701 passed, 1 skipped.
• Focused operation-error and AES-GCM vector tests passed on VM, Chrome Dart2Js, and Chrome Dart2Wasm.
• dart analyze --fatal-warnings . — passed.
• git diff --check upstream/master...HEAD — passed.