fix(attestation): reject oversized challenges and rewrite cert DER encoding

DuckDetector flagged two issues:
1. Oversized challenge accepted, 256-byte attestation challenge should
   return INVALID_INPUT_LENGTH (-21) like real KeyMint. Added early check
   in handleGenerateKey before any path decision.
2. Issuer/subject chain mismatch, rcgen's HashMap loses DN attribute
   ordering and converts PrintableString to UTF8String, producing
   different DER bytes. Replaced rcgen with manual DER assembly that
   injects raw keybox issuer_dn_der bytes directly.

Verified on device: TX_ID 315 rejects 256-byte challenge, TX_ID 501
generates valid 4-cert chain with correct issuer linkage.

native-certgen/Cargo.toml

@@ -13,7 +13,6 @@ ring = "0.17.14"
 rsa = { version = "0.9", features = ["sha2"] }
 pkcs8 = { version = "0.10", features = ["alloc"] }
 rand = "0.8"
-rcgen = { version = "0.13.2", default-features = false, features = ["ring"] }
 der = { version = "0.7.10", features = ["alloc", "oid"] }
 const-oid = "0.9.6"
 x509-cert = { version = "0.2.5", features = ["pem"] }