test: add comprehensive coverage for quic/tls.py (#497)

Add 32 tests covering the full libp2p TLS certificate generation module
from 0% to 100% statement and branch coverage.

Test groups:
- Protocol constants (OID, signature prefix, key type)
- ASN.1 DER length encoding (all 3 branches: short, 1-byte long, 2-byte long)
- ASN.1 OCTET STRING and SEQUENCE wrappers
- ASN.1 SignedKey structure validation
- Extension payload protobuf encoding and signature verification
- Full certificate pipeline (P-256 key, empty DN, SHA-256, self-signed
  verification, validity window with 1-day/365-day tolerance, SKI,
  libp2p extension, ephemeral key uniqueness, PEM round-trip)

Closes #491

Author: latifkasuli