feat(appkit): add testing-kit helpers for env, errors, context, and cache

[IamGalymzhan]

Stack

• #530 — the testing kit (merged)
  • #540 — createTestApp and the mock client
    • #555 — env, error, context, and cache test helpers ← you are here
      • #542 — the playground smoke tests

Each PR targets the one above it, so the diff here is only the delta on top of #540. Review in order.

---

Summary

Adds customer-facing helpers to @databricks/appkit/testing so plugin tests assert business logic instead of hand-rolled boilerplate, then adopts them across the existing suites.

New helpers

• withEnv(vars, fn) — set env for the duration of fn, then restore each key's prior state (deleting keys that were unset before). Sync and async forms; nested calls restore LIFO. Replaces the process.env.X = …; try/finally; delete pattern, which wrongly drops pre-existing env.
• createTestPluginContext(fakes, { responses, env, strict }) — a synchronous options parameter that composes a plugin unit-test in one call: a mock workspace client from responses, an auto-restored service context wired to it, and scoped env. Non-breaking — calls with no second argument are unchanged.
• createApiError({ statusCode, message, errorCode }) — a genuine ApiError (so error instanceof ApiError holds) for error-path tests.
• useTestCache() — boots AppKit's real in-memory cache, clears it before each test, and returns the real CacheManager, so a plugin's caching path runs through production's own getOrExecute/generateKey instead of a mocked cache module. CacheManager stays a process singleton.

Adoption

• The analytics, metric, and ai-search suites drop their vi.mock("cache") fakes and assert against the real cache via useTestCache().
• 17 passthrough suites share a new internal createCacheMock() in place of a copy-pasted fake.
• withEnv replaces 11 hand-rolled env blocks, and createMockRouter is surfaced in the testing guide.

The internal-mock migration and the singleton-reset changes stay deferred to a later PR.

Stacked on #540 (feat/testing-kit-harness) — these helpers extend testing-kit code on that branch, so this targets it rather than main.