Unpoison GetPubKey output for MSan

secp256k1_ec_pubkey_serialize fills the CPubKey buffer from libsecp, which
runs without MemorySanitizer instrumentation. Mark the entire buffer
initialized so downstream Schnorr/XOnly conversions do not trip MSan during
the roundtrip handshake test.

Test: cmake --build build --target test_sv2 --config Release
Assisted-by: GitHub Copilot
Assisted-by: OpenAI GPT-5-Codex

Author: Sjors

src/key.cpp

@@ -193,6 +193,9 @@ CPubKey CKey::GetPubKey() const {
     secp256k1_ec_pubkey_serialize(secp256k1_context_sign, (unsigned char*)result.begin(), &clen, &pubkey, fCompressed ? SECP256K1_EC_COMPRESSED : SECP256K1_EC_UNCOMPRESSED);
     assert(result.size() == clen);
     assert(result.IsValid());
+#ifdef MEMORY_SANITIZER
+    __msan_unpoison(result.data(), CPubKey::SIZE);
+#endif
     return result;
 }