fix(addDummyToLef): prevent fd leaks and add input file validation with clear error handling

[ashnaaseth2325-oss]

1. SUMMARY

This PR fixes unsafe file handling in flow/util/addDummyToLef.py by replacing manual open()/close() usage with context managers and adding an input file existence check. It ensures proper resource cleanup and provides clear error messages when the input LEF file is missing.

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2. FIX

# Before
f = open(args.inputLef)
content = f.read()
f.close()

# After
if not os.path.isfile(args.inputLef):
    print(f"Error: Input LEF not found: {args.inputLef}", file=sys.stderr)
    sys.exit(1)

with open(args.inputLef) as f:
    content = f.read()

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3. VERIFICATION

Run the script with an invalid --inputLef path and confirm it exits cleanly with a clear error message instead of a traceback. Then run it with a valid LEF file and verify the output is generated correctly with no behavior changes. Also confirm no file descriptor issues occur during read/write failures.

[ashnaaseth2325-oss]

Hi @maliberty @eder-matheus,
Kindly review this PR.
Thanks!

[ashnaaseth2325-oss]

Hello @maliberty
Thanks for the approval! I pushed a small fix to address the failing checks. CI is running again.
Please feel free to re-enable auto-merge once everything passes.

[ashnaaseth2325-oss]

Hello @maliberty
Gentle follow-up in case this was missed.