fix(test): isolate mockCurrentUser in announcement-admin-route tests

[garvitkaushik-123]

Summary

Eliminate thread-safety issues in test files that run under the root vitest config (pool: 'threads') by isolating shared mutable state. This addresses root cause (3) from the #6740 triage and hardens all other process.env mutation sites found in the tests/ tree against cross-thread bleed.

Root causes (1) and (2) were fixed in #6800.

Changes

tests/announcement/announcement-admin-route.test.ts — shared mutable mock object

• Replace mockCurrentUser shared object with mockGetCurrentUser vi.fn() factory
• Auth middleware mocks call mockGetCurrentUser() per invocation instead of sharing a reference
• 403 test uses mockReturnValue() instead of property mutation

tests/announcement/announcement-channel-resolver.test.ts — env var leak

• Replace manual process.env delete/restore with vi.stubEnv + vi.unstubAllEnvs
• Remove afterEach and ORIGINAL_ENV manual save/restore pattern
• SLACK_EDITORIAL_REVIEW_CHANNEL mutations no longer leak to concurrent workers

tests/addie/admin-tools.test.ts — env var leak

• Replace unconditional process.env.ADMIN_API_KEY / process.env.BASE_URL assignments with vi.stubEnv
• Add vi.unstubAllEnvs() in beforeEach for automatic restoration

tests/billing/webhook-security.test.ts — env var leak

• Replace direct process.env.STRIPE_WEBHOOK_SECRET / STRIPE_SECRET_KEY assignments with vi.stubEnv
• Replace manual delete/restore in "webhook secret must be configured" test
• Add vi.unstubAllEnvs() in beforeEach for automatic restoration

Verification

• npx vitest run tests/announcement/ — 175/175 passed
• npx vitest run tests/addie/admin-tools.test.ts — 30/30 passed
• npx vitest run tests/billing/webhook-security.test.ts — 7/7 passed
• Full parallel run (12 files, 212 tests) — all passed under threads pool
• Swept all tests/ files for remaining non-idempotent process.env mutations — zero remaining

Refs #6740

[aao-secretariat]

Ladon verdict: Approve

Approve — test-only change with no blocking or medium findings.

This PR touches a single test file (tests/announcement/announcement-admin-route.test.ts, +7/−6), replacing a shared mutable mockCurrentUser object with a mockGetCurrentUser vi.fn() and setting return values via mockReturnValue instead of mutating a shared reference across sequential requests. The fix eliminates in-place mutation of shared state and is self-consistent.

No protocol surface (static/schemas/source/**), docs, changeset, or released-artifact scope is touched — schema↔docs coherence, changeset, oneOf, and immutability gates do not apply. No no-auto-approve team match, gated_paths is false, and high_risk is false.

Decision table: rows 1–8 do not fire (no critical/high, no medium, no gated paths, no deletions, no team gate). Falls through to row 9 → approve. Note review_decision is REVIEW_REQUIRED, but with gated_paths: false and no other trigger, that does not block approval.

[aao-secretariat]

Re-approval: no reviewable delta since the prior approval; re-submitting after stale-review dismissal.