oidmap: make entry cleanup explicit in oidmap_clear

oidmap: make entry cleanup explicit in oidmap_clear

Replace oidmap's use of hashmap_clear_() and layout-dependent freeing
with an explicit iteration and optional free callback. This removes
reliance on struct layout assumptions while keeping the existing API
intact.

Add tests for oidmap_clear_with_free behavior.
test_oidmap__clear_with_free_callback verifies that entries are freed
when a callback is provided, while
test_oidmap__clear_without_free_callback verifies that entries are not
freed when no callback is given. These tests ensure the new clear
implementation behaves correctly and preserves ownership semantics.

Signed-off-by: Seyi Kuforiji <kuforiji98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>