fix: return full keybox chain when BYO attest key misses

CertificateGenerator.generateCertificateChain selected
keybox.keyPair as fallback signer when getAttestationKeyInfo returned
null, but the chain assembly at line 115 still keyed on
"attestKeyAlias != null" and returned only listOf(leafCert). The
caller received a depth-1 chain signed by the keybox root with no
parent attached, structurally invalid.

Track whether the BYO lookup actually returned a key. On hit return
the depth-1 chain (caller holds the rest). On miss include
keybox.certificates so the chain is rooted.

Surfaced by adversarial audit of f384871, which broadened the
software-dispatch gate to all attestationKey != null requests.
Without this companion fix the miss path produces a malformed chain
where the previous code would have forwarded to HAL.

app/src/main/java/org/matrix/TEESimulator/pki/CertificateGenerator.kt

@@ -101,18 +101,19 @@ object CertificateGenerator {
 
                 val keybox = getKeyboxForAlgorithm(uid, params.algorithm)
 
-                val (signingKey, issuer) =
+                val attestKeyInfo =
                     if (attestKeyAlias != null && Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
-                        getAttestationKeyInfo(uid, attestKeyAlias)?.let { it.first to it.second }
+                        getAttestationKeyInfo(uid, attestKeyAlias)
+                    } else null
+
+                val (signingKey, issuer) = attestKeyInfo
+                    ?.let { it.first to it.second }
                     ?: (keybox.keyPair to getIssuerFromKeybox(keybox))
-                    } else {
-                        keybox.keyPair to getIssuerFromKeybox(keybox)
-                    }
 
                 val leafCert =
                     buildCertificate(subjectKeyPair, signingKey, issuer, params, uid, securityLevel)
 
-                if (attestKeyAlias != null) {
+                if (attestKeyInfo != null) {
                     listOf(leafCert)
                 } else {
                     listOf(leafCert) + keybox.certificates