THRIFT-5926: Fix TSaslClientTransport.open() for DIGEST-MD5

[ShreyeshArangath]

Summary

This PR fixes multiple critical failures in TSaslClientTransport.open() when using challenge-response SASL mechanisms (like DIGEST-MD5). These issues previously led to TypeErrors, failed mutual authentication, and protocol desynchronization.

Here are some of the problems:

1. Mechanisms where the server speaks first like DIGEST-MD5 cause sasl.process() to return None. Passing this to send_sasl_msg triggered len(None), killing the connection instantly.
2. The client was ignoring the final server challenge in the COMPLETE message. This prevented the client from verifying the server’s identity (rspauth) and left the SASL state machine in an incomplete state.
3. recv_sasl_msg returned a string ("") instead of bytes (b"") for zero-length payloads, risking downstream failures in puresasl.

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