…le loaders (#3663)

## Summary

- Dashboard loaders for runs / sessions / batches / schedule-detail
threw bare `Error("X not found")` when a slug didn't resolve. Remix
surfaces this as a 500 and Sentry captures it via auto-instrumentation,
producing ongoing noise from real users following stale preview-branch
or deleted-resource links (the URLs in those Sentry events all carry
`?_data=routes/...`, i.e. client-side revalidation, not full-page
navigation).
- Added a `throwNotFound(statusText)` helper in
`app/utils/httpErrors.ts` that throws a Response with status 404,
matching the established pattern in sibling routes (agents, alerts,
bulk-actions, etc.).
- Migrated 5 loader sites to `throwNotFound` (4× "Environment not
found", 1× "Schedule not found").
- Migrated 1 loader site (`runs._index` project branch) to
`redirectWithErrorMessage("/", request, "Project not found")` to match
the pre-existing convention used by every other dashboard route's
project-not-found branch.
- Intentionally **not** touched: bare `throw new Error("X not found")`
inside `resources.*` action routes (sit inside try/catch blocks that
already redirect with a flash message), the invariant assertion in
`vercel.connect.tsx`, and the admin config check in
`admin.api.v1.runs-replication.backfill.ts`.

## Where the fix is visible

Normal browser navigation to these URLs doesn't reach the buggy loaders
— the parent env-layout
(`_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam/route.tsx`)
already filters missing envs/projects and redirects/404s before the
child loader runs. The bug fires exclusively when Remix calls a single
child loader via `?_data=routes/...`, which happens during client-side
navigation or `useRevalidator`. That matches every Sentry event URL.

## Test plan

- [x] Unit test for the new helper —
`apps/webapp/test/httpErrors.test.ts`
- [x] `pnpm run typecheck --filter webapp` clean
- [x] Manual verification via Playwright on `main` vs this branch (6
cases): main returns 500 for each defective `_data` URL; branch returns
404 or 204 + `X-Remix-Redirect` as designed
- [x] Verified user-visible 404 catch boundary on `schedules/<missing>`
(the one case reachable via normal nav)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>