feat(genai): Collapse and expand a whole conversation - #4459

#### Which problem is this PR solving?

jaegertracing#4425 gave each message its own collapse toggle, which covers the case where
the reader already knows which message is in the way. It left no way to act on
the section as a whole: returning a folded conversation to normal means
expanding message after message, and folding a long one means the work scales
with the size of the problem.

Part of jaegertracing#4427.

#### Description of the changes

The Conversation section header gains a Collapse all / Expand all pair, shown
once there are at least two messages for it to act on. Below that the message's
own toggle already is the pair. Both controls are absolute: each writes every
message, so the section afterwards is entirely folded or entirely open rather
than partly either.

Making that possible moves the folded flag out of MessageBlock and into
ConversationDetails, held as a record keyed by the key each row already carries
(system, input-N, output-N). A message still owns its toggle, it just no longer
owns the answer, so the section's control and the per-message button cannot
disagree. MessageBlock's button, its aria-expanded contract and its collapsed
preview branch are otherwise unchanged.

The record is seeded in a lazy initializer rather than an effect. The messages
array is rebuilt on every render, so an effect keyed on it would refire on each
new array identity and discard whatever the reader had folded.

Neither bulk control reports aria-expanded. Neither owns a region of its own,
and a third expandable button would also make the per-message expanded counts
ambiguous to a screen reader.

Nothing is persisted. The state lives with the mounted tab, the same as the
per-message toggle and the format choice beside it.

Left out deliberately, and still open in jaegertracing#4427: any size threshold or
auto-collapse for a long conversation. That needs a decision about what counts
as long and which messages should start folded, and it is a separate change.

#### How was this change tested?

Five tests added to the existing collapsing block: collapse all folds every
message, expand all opens every message including ones folded individually, an
individual toggle still works after each bulk action, and the pair is absent
for a single-message conversation. The existing per-message assertions, which
count buttons by aria-expanded, still hold unchanged.

Full suite 3822 passing across 280 files, up 5 from 3817 on main. oxlint exits
0 with the same 311 pre-existing warnings, and tsc-lint is clean.

Claude-Session: https://claude.ai/code/session_01WPkifMsD3qrCAjZ5gLidPZ
Signed-off-by: Utkarsh Bahuguna <utkarshbahuguna10@gmail.com>