fix: account for operation polling timeout

[pragnyanramtha]

Summary

• Fix long-running operation polling timeout accounting so pending operations eventually raise after the configured timeout.
• Add a focused transformer unit test that catches the previous infinite-polling behavior without sleeping in real time.

Why

t_resolve_operation initialized total_seconds to 0.0 but updated it with total_seconds += total_seconds, which leaves it at zero forever. If an operation never reaches done: true, the timeout guard never fires.

Validation

• .venv/bin/python -m pytest google/genai/tests/transformers/test_t_operation.py -q
• .venv/bin/python -m pytest google/genai/tests/transformers/test_t_*.py -q
• .venv/bin/python -m py_compile google/genai/_transformers.py google/genai/tests/transformers/test_t_operation.py
• git diff --check

Note: pytest google/genai/tests/transformers -q could not collect in this local Python 3.14 environment because google/genai/tests/transformers/test_blobs.py imports PIL, and installing pinned Pillow==11.0.0 from requirements.txt failed due missing JPEG headers/no compatible wheel in this image.