test(rest): REST client test suite

[0xSaiNova]

Closes #6

The REST client had no real test coverage. This adds four new test files and expands the existing one to cover everything the issue asked for.

Route bucket keys

Route.test.ts is new. Verifies that major params (server ID, channel ID) produce separate rate limit buckets while minor params (message ID) get normalized so requests against different messages in the same channel share one.

Error class hierarchy

errors.test.ts is new. Every error class gets tested for the right inheritance chain and the fields specific to it. RateLimitError gets extra attention since retryAfter, global, and bucket are the whole point of that class existing.

Request basics

REST.test.ts now stubs global.fetch inside a scoped describe block to verify that REST actually sends the right method, URL, auth header, and body. The stub is cleaned up after each test so it doesn't affect the existing token guard tests.

Error status mapping

429, 401, 403, 404, 500, 502, and unknown 4xx each get a test asserting the right error type comes back. The 429 and 5xx tests use maxRetries: 0 so they throw immediately rather than sleeping through retry delays and exhausting the fetch mock queue.

Snowflake validation

The original tests only covered getServer. This expands to getChannel, createMessage, getMessage (both IDs separately), and the before/after cursors on listMessages. Also adds a positive case confirming a valid numeric snowflake passes through to the network layer.

[0xSaiNova]

Worth noting the approach for anyone reviewing: fetch is stubbed inside a scoped describe block with beforeEach/afterEach so it only affects the tests that need it. The existing token guard tests make real network calls (or rather, let them fail naturally) and they still work fine.

describe('request basics', () => {
  const fetchMock = vi.fn()

  beforeEach(() => {
    vi.stubGlobal('fetch', fetchMock)
    fetchMock.mockReset()
  })

  afterEach(() => {
    vi.unstubAllGlobals()
  })

  // ...
})

The maxRetries: 0 pattern on the 429 and 5xx tests is intentional. Without it, REST retries those errors with delays and the mock queue runs out, causing the retry to get undefined back from fetch instead of a proper response.