The #7904 fix stopped masking provider errors as "Handoff cancelled", but
an empty or whitespace-only generation still fell through: whitespace-only
text passed the `!handoffText` guard and produced a bogus handoff, while
empty text returned undefined which the interactive /handoff caller mapped
to "Handoff cancelled" with no detail and no log entry.
Treat empty/whitespace-only output as a real failure: a user-initiated
handoff throws "Handoff generation produced no content" (surfaced as
"Handoff failed: ...") and logs it; auto-handoff keeps returning undefined
so maintenance falls back to context-full compaction. Also log genuine
handoff failures in the command controller so they persist for debugging.
Fixes#7993
roboomp
92e574cb02