fix(hitl): show mapped task instances on /required_actions listing

[paultmathew]

Fixes #66428.

Why

/required_actions and the per-DAG / per-Run / per-Task Required Actions
tabs all silently dropped HITL rows for mapped task instances because the
listing component hard-coded mapIndex: parseInt(searchParams.get(MAP_INDEX) ?? "-1", 10),
which always sent map_index=-1 and filtered to non-mapped TIs only.
Issue #66428 has the full repro and access-log evidence.

What

• Pass mapIndex to the listing call only when the user has explicitly
  set the map_index URL search param. Default behaviour is now
  "all map indexes" (no filter) instead of "non-mapped only", and
  map_index=-1 continues to work as an opt-in filter for non-mapped
  task instances.
• Fix the auto-refresh predicate that compared
  responded_at === undefined instead of === null. The API serializes
  responded_at as JSON null, never an omitted field, so the predicate
  never matched and the page never polled for new pending actions while
  open. The issue's "Anything else" section flagged this; the OP offered
  to roll it into the same PR and there has been no maintainer pushback,
  so it is included here.

Tests

• Added HITLTaskInstances.test.tsx covering the three URL-param
  branches (map_index absent / set to 2 / set to -1) and the
  refetch predicate for both responded_at: null and a populated value.
• Locally verified with pnpm exec vitest run src/pages/HITLTaskInstances/HITLTaskInstances.test.tsx
  (5/5 passing), pnpm exec eslint src/pages/HITLTaskInstances/,
  pnpm exec tsc -p tsconfig.app.json --noEmit, and prek run --files
  on the changed files.

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