fix: add ApiClient.close() to clean up ThreadLocal state on thread reuse

[prachi-okta]

Problem

ApiClient stores last response state in static ThreadLocal fields for backward compatibility with PaginationUtil. In thread-pool environments, threads are reused across requests, causing stale headers to leak between requests.

Changes

• Added close() to clear lastStatusCode and lastResponseHeaders ThreadLocals for the current thread
• Added unit tests in ApiClientTest (cleanup, idempotency, thread isolation)

Note: Only affects the deprecated PaginationUtil path — the new PagedIterable is already stateless.