[Fix] Isolate Slack install-after-auth route tests from developer .env.local

[mrubens]

Problem

apps/web/src/app/api/slack/install-after-auth/__tests__/route.test.ts fails locally for any developer whose repo-root .env.local sets R_PUBLIC_URL (e.g. an ngrok URL). The test stubbed process.env.R_APP_URL/R_PUBLIC_URL (isolation added in #758), but the route resolves the public URL via getCallbackHost → the dotenvx-backed web Env proxy, which reads the repo-root .env.local file directly — so the file value bypasses the process.env stubs. Symptom: expected http://localhost:13000/... but received https://<ngrok-domain>/... in the two redirect tests.

Fix

Replace the process.env stub/restore dance with the existing repo pattern (vi.hoisted env state + vi.mock('@/lib/server/env'), as in trpc/commands/linear/index.test.ts): spread the real module and override Env with a test-owned object, so dotenvx file resolution is never consulted. getPublicAppUrl stays unmocked so the real R_PUBLIC_URL ?? R_APP_URL preference logic remains under test.

Verification

pnpm exec dotenvx run -f .env.test -- pnpm --filter web exec vitest run src/app/api/slack/install-after-auth/__tests__/route.test.ts

• Reproduced the failure first with a repo-root .env.local setting R_PUBLIC_URL=https://example-dev.ngrok.app (2/3 failed)
• After the fix: 3/3 pass both with and without that .env.local present
• pnpm lint and pnpm --filter web check-types clean

[roomote-community]

No code issues found. See task

_{Reviewed 3b58745}