[v0.1.x-branch] Backport #1007: multi: Report empty round-join as no-op, not INTERNAL

[github-actions]

Backport of #1007

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In this PR, we fix a third bug found while manually testing #987 on the
arktest regtest harness: a refresh or leave that queued nothing still
auto-joins the next round, and that join fails with a confusing
INTERNAL error instead of a clean no-op.

The bug

ark vtxos refresh --all (or leave --all) with no live VTXOs — or any
selection that resolves to nothing — correctly returns a queued no-op
from RefreshVTXOs. But the CLI then auto-joins the next round, and
JoinNextRound fails:

INTERNAL: join next round: failed to trigger round registration:
no pending round for event *round.IntentRequested

The command exits 1 with an error that names neither the cause nor the
fix, even though nothing actually went wrong — there was simply nothing
to join.

Observed on the live daemon: refresh --all --yes with an empty wallet
prints the queued no-op JSON, then the INTERNAL above; with this
change it prints nothing queued to join and exits 0.

The fix

The round actor produced the "no pending round" condition as a bare
fmt.Errorf, so no caller could tell this benign "nothing was queued"
case apart from a genuine internal fault. We give it a typed
ErrNoPendingRound sentinel, wrapped with %w through
TriggerRoundRegistration. JoinNextRound now maps it to a clean
JoinNextRoundResponse{Status: "nothing_to_join"} instead of
codes.Internal, and the CLI's auto-join step prints nothing queued to join rather than surfacing an error. An explicit ark rounds join with
nothing queued gets the same clean treatment.

Validation

• New round test: a server message with no pending round fails with a
  typed error (errors.Is(..., ErrNoPendingRound)).
• New cmd/wavecli tests: maybeJoinNextRound reports nothing_to_join
  as a no-op, and an ordinary join still prints the join notice.
• Full round and cmd/wavecli/waveclicommands packages under -race.
• make fmt-changed-check, make lint-changed-local (0 issues),
  make commitmsg-lint, make build.
• Live re-verification on the arktest harness (empty --all --yes now
  reports the no-op cleanly).

Found while manually testing #987.