fix: parse mldev batch input config source

[MukundaKatta]

Fixes #2367.

Summary

• parse metadata.inputConfig into BatchJob.src for Gemini Developer API batch responses
• add a converter regression test for MLDev batch input and output file fields

Tests

• uv run --with pytest pytest google/genai/tests/batches/test_batch_converters.py -q
• git diff --check

[Venkaiahbabuneelam]

Hi @MukundaKatta, Thanks for reaching out us!

Some checks have failed, please resolve the conflicts.

Thanks