fix(showcase/aimock): drop stale turnIndex:0 gate from multi-turn D6 pill fixtures

D6 drives each feature's pills as sequential turns in one chat thread.
aimock matches turnIndex against the count of assistant messages in
history, so a "turnIndex": 0 gate on a per-pill tool-call leg only
matches the FIRST pill — pills 2+ (turnIndex 1/2/3) match no fixture,
503 under strict, and the harness reports "timeout: assistant did not
respond".

Remove the turnIndex:0 gate from the multi-turn tool-call legs of
frontend-tools (sunset/forest/cosmic) and tool-rendering-reasoning-chain
(the three chained pills: Compare AAPL/MSFT, compare-to-smaller dice,
weather-there flights) so they match on their distinct userMessage
substrings + context, mirroring the already-green langgraph-python
fixtures. Each pill's substring is unambiguous and its toolCallId-keyed
follow-up fixture is ordered before the de-gated first leg (first-match
wins), so follow-up narration still wins and there is no cross-matching.

The reasoning-chain "Find flights from SFO to JFK." pill keeps its
turnIndex:0 gate to match the green langgraph-python reference exactly.

Proven locally with aimock --strict: multi-turn requests for these pills
returned 503 with the gate (negative control) and now return 200 across
all turns with the correct fixture/narration.