fix(retry): ensure Retry Source CID differs from client DCID

[MarkedMuichiro]

Fixes #5302

Problem

When generating a Retry packet, CxPlatRandom produces a random Source Connection ID but does not check that it differs from the client's Destination CID. RFC 9000 §17.2.5.1 requires that the Source Connection ID in a Retry packet MUST NOT equal the Destination Connection ID sent by the client.

While a collision is astronomically unlikely, the check is required for RFC compliance.

Fix

Wrapped the CxPlatRandom call in a do-while loop that regenerates if the new CID matches the client's DCID:

do {
    CxPlatRandom(MsQuicLib.CidTotalLength, NewDestCid);
} while (RecvPacket->DestCidLen == MsQuicLib.CidTotalLength &&
         memcmp(NewDestCid, RecvPacket->DestCid, MsQuicLib.CidTotalLength) == 0);

The length guard is load-bearing — the client DCID length is client-controlled and can legitimately differ from CidTotalLength, so skipping it would risk comparing against a shorter buffer.

This also fixes a minor issue in the original: CxPlatRandom was called with sizeof(NewDestCid) which randomizes the full QUIC_CID_MAX_LENGTH buffer, but only MsQuicLib.CidTotalLength bytes are ever used downstream. The fix randomizes only the bytes that matter.