Constrata's coordinator transit engine `ciphertextContainer.UnmarshalJSON` panics on attacker-controlled short ciphertexts

## Summary `ciphertextContainer.UnmarshalJSON` decodes the third `:`-separated component of a `vault:vX:base64...` ciphertext and then unconditionally takes a 12-byte prefix slice for the AES-GCM nonce: `c.nonce = fullCiphertext[:aesGCMNonceSize]`. If the decoded blob is shorter than 12 bytes, the slice expression panics. The panic happens before any cryptographic operation, while the JSON body of the request is still being parsed inside the request handler. Because the handler is invoked from `net/http`'s standard handler goroutine, the panic is recovered to a 500 response, but the request handler aborts mid-execution and the recovered panic appears in the Coordinator's logs. An authenticated workload that holds a valid mesh certificate for any `WorkloadSecretID` can trigger the panic at will, producing log spam, request-failure metrics, and a slow but cheap denial of service against the transit-engine endpoint. ## Details ### the panicking slice `coordinator/internal/transitengineapi/crypto.go:64-88`: ```go // UnmarshalJSON umarshalls a json string to a ciphertextContainer holding the version prefix, // decoded base64 nonce and ciphertext. func (c *ciphertextContainer) UnmarshalJSON(data []byte) error { var encoded string if err := json.Unmarshal(data, &encoded); err != nil { return err } // Split "vault:vX:base64" format parts := strings.SplitN(encoded, ":", 3) if len(parts) < 3 { return fmt.Errorf("invalid ciphertext format") } version, err := extractVersion(parts[1]) if