fix: poll(timeout_ms=0) returns buffered records when coordinator times out

[HKanaparthi]

Fixes #2692

Root cause:
When timeout_ms=0, coordinator.poll() returns False immediately, causing
an early return {} at line 722 that skipped the fetched_records() buffer
check — so buffered messages from previous poll calls were silently dropped.

Fix:
Added a buffer check inside the early-exit branch so already-fetched
records are still returned even when the coordinator times out.
Non-blocking behavior is fully preserved.

Tests:
Added 2 regression tests covering:

• poll(timeout_ms=0) returns buffered records when coordinator times out
• poll(timeout_ms=0) returns empty when no records are buffered

Note: Tests use mocks. Happy to add integration tests if needed.