When sanitize_agent_error is called with both exc and stderr, the exc
class name was leaking into the user-visible message even though stderr
already provides actionable context. Only include the tag when an
explicit category is supplied; fall back to the bare form when the
tag would have come from type(exc).__name__.
Fixes test_sanitize_agent_error_stderr_and_exc regression introduced
in commit 7290d9727.
