What issue does this PR close?

Partially address #1749.

Rationale for this change

This PR fixes a bug in delete file loading when a FileScanTask contains both positional and equality delete files. We hit this when running Iceberg Java test suite via Comet in apache/datafusion-comet#2528. The tests that failed were

TestSparkExecutorCache > testMergeOnReadUpdate()
TestSparkExecutorCache > testMergeOnReadMerge()
TestSparkExecutorCache > testMergeOnReadDelete()

The Bug:
The condition in try_start_eq_del_load (delete_filter.rs:71-73) was inverted. It returned None when the equality delete file was not in the cache, causing the loader to skip loading it. When build_equality_delete_predicate was later called, it would fail with "Missing predicate for equality delete file".

What changes are included in this PR?

The Fix:

• Inverted the condition so it returns None when the file is already in the cache (being loaded or loaded), preventing duplicate work across concurrent tasks
• When the file is not in the cache, mark it as Loading and proceed with loading

Additional Changes:

• Added test case test_load_deletes_with_mixed_types that reproduces the bug scenario

Are these changes tested?

Yes, this PR includes a new unit test test_load_deletes_with_mixed_types that:

• Creates a FileScanTask with both a positional delete file and an equality delete file
• Verifies that load_deletes successfully processes both types
• Verifies that build_equality_delete_predicate succeeds without the "Missing predicate" error
• We hit this when running Iceberg Java test suite via Comet in feat: [iceberg] Native scan by serializing FileScanTasks to iceberg-rust datafusion-comet#2528. I also confirmed that it fixes the tests in Iceberg Java's suite.

The test would fail before this fix and passes after.