default to merge append instead of fast append

devlist discussion: https://lists.apache.org/thread/sk1j4jo6s7ntr880tt7ktdt636nwzt5n

Merge append is better for metadata management

Start with the linked devlist discussion to understand why merge append should become the default and what metadata-management behavior is expected. Then locate the append entry point and its current fast-append default in the PyIceberg codebase. Done means the default uses merge append and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.