Add tests for retry-after HTTP-date values #5

Background:
The retry behaviour supports retry-after values as either seconds or HTTP-date
strings. Existing tests cover retry behaviour broadly; adding a focused HTTP-date
case would make this behaviour easier to protect.

Proposed change:
Add a unit test that returns a 429 response with an HTTP-date retry-after
header and verifies the retry path completes with a successful follow-up
response.

Acceptance criteria:

• A test covers an HTTP-date retry-after value.
• The test uses a deterministic short wait so the suite stays fast.
• The test verifies that the final successful response is returned.
• bun run test passes.

Files likely involved:

• src/__tests__/client.test.ts
• src/client/BaseHttpClient.ts for source reference only