fix(devkit): avoid dtype warning for failed outputs

[fengjikui]

Summary

• Fill missing failed output values after converting output columns to object dtype.
• Preserve the line-number column while avoiding pandas incompatible-dtype FutureWarning.
• Add a regression test for numeric output columns with padded failed rows.

Root cause

When _outputs_padding adds missing output rows, numeric output columns contain NaN. Filling the whole dataframe with the string "(Failed)" attempts to write a string into a float column, which triggers pandas' incompatible-dtype FutureWarning and will become an error in a future pandas release.

Fixes #2702

Validation

• src/promptflow-evals/.venv/bin/python -m pytest src/promptflow-devkit/tests/sdk_cli_test/unittests/test_local_storage_operations.py -q (2 passed)