fix(task-board): don't count unpriced runs in the task-cost run count

[pedrofrxncx]

Follows #6154/#6159 (the task-cost chip and its plural fix).

Bug: TaskCost in task-dialog.tsx summed cost only over threads with a non-null costUsd, but reported the run count as (threads ?? []).length — every linked thread, including ones with no finish part yet (still running, or crashed before completing one, per parseCostUsd/the cost lateral join in apps/api/src/storage/task-board.ts). A task with one priced run and one still-running thread showed "$X in 2 runs", but the total only ever covered the one priced run — the tooltip explicitly claims the total is "across all {runs} of its runs", which was false.

Fix: extracted the count/total logic into a pure summarizeTaskCost() helper (task-cost.ts) and made the run count priced.length instead of the full thread list, so the displayed count always matches what the total actually sums.

Regression test: task-cost.test.ts — summarizeTaskCost([priced, unpriced, priced]) now asserts runCount: 2, not 3.

To confirm: bun test apps/web/src/layouts/task-board/task-cost.test.ts

Locally verified: bun run fmt, bunx tsc --noEmit in apps/web (one pre-existing, unrelated mustache type error on main, nothing in the touched files), bunx oxlint on the three changed files (0 warnings/errors), and the targeted test above. Full CI validates the rest.

---

Summary by cubic

Fixes the task board cost chip to count only runs that recorded costUsd, so the run count matches the summed total. Previously it summed priced threads but reported the count of all threads, including still-running or crashed ones.

• Extracts summarizeTaskCost() in task-cost.ts and uses it in TaskCost; the chip is hidden when no priced runs exist.
• Adds task-cost.test.ts to cover exclusion of unpriced threads and null cases; task-dialog.tsx is refactored to consume the helper.

^{Written for commit 7615a9a. Summary will update on new commits.}