Improve first-attempt ask consistency

[variablenix]

Summary

• bound each !ask provider so a slow Search Assist request cannot consume the full lookup window
• retry transient DuckDuckGo Instant Answer responses within the same invocation
• wait for rendered DuckDuckGo result cards and reuse one captured browser result across fallback decisions
• cache successful normalized questions briefly to avoid repeated provider races
• add regression coverage and document cache_seconds

Verification

• go test ./...
• go vet ./...
• go test -race ./plugins
• git diff --check
• go build -o /tmp/GoBot-irc-bot-build ./cmd/irc-bot