Fix partial socket reads causing ‘Failed reading packet’ errors

[Phlogi]

• Fix partial recv() reads by adding read_exact() and using it for header/payload/MAC.
• Explicit EOF handling so clean disconnects don’t look like corruption.
• Keep MAC mismatch distinct for clearer debugging.
• This resolves the recurring log line: CRITICAL:Librespot:Session:Failed reading packet! Failed to receive packet