fix: ensure element.out is always created in interstitial autotest

[Copilot]

Problem

Some autotest interstitial jobs were failing when using conf_filters with min_dist constraints that filter out all generated defects. The issue occurred because:

1. When all interstitial defects are filtered out by distance constraints, the element.out file is never created
2. Later processing stages (especially post_process) expect this file to exist and fail when it's missing
3. This causes dpgen autotest make to crash with errors like attempting to read from a non-existent file

Root Cause

The element.out file was only created when defects were actually written to it during the generation loop. If conf_filters removed all defects, the file was never opened/created, leaving subsequent processing steps unable to find the expected file.

Solution

This PR implements a minimal fix with two key changes:

1. Always Create element.out

# Always create element.out file, even if it will be empty
with open(insert_element_task, "w") as f:
    pass

The file is now created immediately after cleanup, ensuring it exists even when no defects pass the filters.

2. Robust post_process Handling

Enhanced the post_process method to gracefully handle:

• Empty task lists
• Missing element.out files
• Mismatched number of element entries vs tasks
• Empty element lines

Testing

Added comprehensive test cases:

• test_make_confs_filtered_out: Reproduces the original bug scenario with high min_dist filtering
• test_make_confs_partial_filtering: Tests robust handling of mismatched element/task counts
• Verified existing functionality remains intact with all original tests passing

Verification

✅ Unit tests pass (5/5 interstitial tests)
✅ CLI commands work with problematic configurations
✅ Normal workflow remains functional
✅ Integration tests confirm fix works end-to-end
✅ Other autotest functionality unaffected

The fix is surgical and minimal - only 3 lines added to ensure file creation, plus robust error handling in post_process. This resolves the issue while maintaining full backward compatibility.

Fixes #500.

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