Core: Preserve explicit delete_data_file() in _DeleteFiles

[qzyu999]

Closes #3857

Rationale for this change

_DeleteFiles._compute_deletes resets self._deleted_data_files = set() before scanning manifests by predicate. Files added via the inherited delete_data_file() method were silently dropped because the reset discards them before the manifest scan begins.

This means calling delete_data_file() on a _DeleteFiles instance (via update_snapshot().delete()) produces no error and no effect.

Root cause

_compute_deletes rebuilds _deleted_data_files from predicate-matched entries only. Explicit file references added before the computation were lost.

Fix

Preserve the explicit set before resetting, and include those files in the should_delete check alongside predicate evaluation. This is a 6-line production change.

Are these changes tested?

Two new tests covering:

1. Explicit delete_data_file() on a _DeleteFiles instance deletes the file
2. Explicit file deletion works when the table has multiple data files (only the targeted file is removed)

Both pass across all 3 catalog backends (memory, sql, sql_without_rowcount). Existing snapshot and commit-retry tests (169 total) continue to pass.

Are there any user-facing changes?

delete_data_file() on a _DeleteFiles instance now correctly deletes the specified file instead of silently doing nothing.