A single XCLIENT parameter: name=xtext-value.
The value field stores the xtext-decoded string; the wire form
uses xtext encoding where non-printable bytes appear as +XX hex pairs.
Render an AUTH command line with its SASL initial-response
masked as REDACTED. When has_initial_response is false,
the returned line is just the verb and mechanism, matching the wire form
for a command with no initial-response.