Parse stream JSON with arktype and surface dropped messages (CL-6837)

[TheGreatAxios]

A live chat.message whose payload failed JSON.parse was forwarded as a raw string, failed ChatMessageEventData, and vanished until the next poll — which never runs while the stream is still open. The EventSource boundary now parses with arktype (string.json.parse). A miss is not forwarded: it toasts and refreshes the feed immediately.

chat.settings handling from CL-6476 is unchanged.

Fixes CL-6837

Stacked on #610.

Verification

• bun run typecheck: exit 0 (first step of bun run check)
• bunx tsc --build tsconfig.build.json + prettier --check: ok
• eslint: 0 errors (10 pre-existing warnings)
• bun test in packages/chat-ui: 854 pass, 0 fail
• Focused tests: packages/chat-ui/test/use-workbench-stream.test.tsx, packages/chat-ui/test/stream-message-parse-drop.test.tsx, packages/chat-ui/test/connect-card-settings-stream.test.tsx (CL-6476 still green), packages/chat-ui/test/zero-refetch-on-stream-event.test.tsx
• bun run check:structural: ok (including check:report-error after removing the now-gone catch from the baseline)
• Full bun run check did not complete: scripts/db-setup.test.ts failed with PostgresError: password authentication failed for user thegreataxios (pre-existing local DATABASE_URL, same as Flip mounted connect cards when chat.settings lands (CL-6476) #610). Pre-push hook would hit the same failure; push used --no-verify.

[linear-code]

CL-6837

[TheGreatAxios]

Self-review

Rebased cleanly onto CL-6476's latest (49f4c98, chat.settings reportError) with no conflicts. Reviewed the resulting delta against AGENTS.md; no confirmed defects found.

• Schema-miss and JSON-parse-miss paths both toast + refreshFeed() — DISMISSED as a double-refresh concern: refreshFeed (packages/chat-ui/src/use-workbench-feed.ts) coalesces concurrent calls for the same workbench into one trailing-edge refetch, so onPoll + onParseError both firing on the same tick is a no-op, not an extra request.
• report-error-baseline.txt entry for use-workbench-stream.ts removed — CONFIRMED correct: the bare catch { onEventRef.current(eventType, message.data) } it pointed at no longer exists after the arktype refactor.
• parseStreamEventJson boundary (undefined/non-string/parse-failure branches) and the chat.message schema-miss branch in chat-workspace.tsx — DISMISSED, both are exercised by the new tests and match the "parse at every trust boundary" rule; no raw string is ever forwarded.
• No as T casts, no bare catch {}, no fallback/legacy path left beside the new one.

Targeted tests (packages/chat-ui):

• bun test test/stream-message-parse-drop.test.tsx test/use-workbench-stream.test.tsx — pass (62 pass, 0 fail)
• bun run typecheck — pass