Support PipelineTrainer with ServerlessBackend

[Kovbo]

Summary

• extend ServerlessBackend.train to accept the PipelineTrainer train kwargs directly
• translate supported PipelineTrainer/serverless training fields into ExperimentalTrainingConfig
• reject unsupported custom loss_fn_config and adam_params explicitly
• add unit coverage for serverless PipelineTrainer compatibility and payload forwarding

Replaces the relevant serverless PipelineTrainer compatibility work from the stale #623 against current main.

Tests

• uv run ruff format --check src/art/serverless/backend.py src/art/serverless/client.py tests/unit/test_serverless_pipeline_trainer_compat.py
• uv run ruff check src/art/serverless/backend.py src/art/serverless/client.py tests/unit/test_serverless_pipeline_trainer_compat.py
• uv run pytest tests/unit/test_serverless_pipeline_trainer_compat.py -q
• uv run python -m py_compile src/art/serverless/backend.py src/art/serverless/client.py tests/unit/test_serverless_pipeline_trainer_compat.py