fix(seer): Stop drawing an empty reasoning box while Seer works

[ryan953]

While a Seer Explorer response is in flight but has reported nothing yet, the
chat drew a bordered but empty card under the Thinking... header. It is most
visible when the agent goes off to run an autofix, because that can sit for tens
of seconds with nothing to narrate — the panel reads as broken or stuck rather
than as work in progress.

Before, the whole panel was a Thinking... row with an elapsed timer and an
empty rounded box beneath it. After, it is the row on its own.

ThinkingBlock opens its content card only when it is given children, but
ResponseGroup handed it group.map(...). An array is truthy even when every
Fragment inside it renders nothing, so that guard never fired. The blocks are
now gated on hasTrace — the predicate that already decides whether any
reasoning prose, narration, or tool row will actually appear — so the header
stands alone until there is something to put under it.

Hardening ThinkingBlock instead was the other option, and it was not taken:
React cannot ask a subtree whether it rendered anything, and
React.Children.count counts the Fragments, not their output. The caller is
the only place that knows, and hasTrace already encodes it.

The block itself still renders the whole time, so the live tool title, the Seer
spinner, and the elapsed timer are unchanged.

No feature flag — this is on the existing Seer Explorer chat path.

The existing renders a ThinkingBlock placeholder before any trace content arrives test asserted only that the disclosure exists, which passed either way;
it now also asserts the content panel is absent, and that assertion fails
against master.