[fix][train] Defer W&B authentication to SDK

[bvolpato]

Summary

SkyRL checked only WANDB_API_KEY before calling W&B. That rejected credentials stored by wandb login and offline runs that do not need API access. It also replaced W&B's authentication errors with a less useful SkyRL assertion.

Changes

• Remove the environment-only check from training configuration validation.
• Remove the same check from WandbTracker.
• Let wandb.init select and validate supported authentication modes.

WANDB_API_KEY behavior remains unchanged. Netrc and offline authentication now follow standard W&B SDK behavior.

Validation

• Config and logging tests (147 passed)
• Regression coverage for config validation and WandbTracker without WANDB_API_KEY
• Training run authenticated through netrc and synchronized metrics successfully
• pre-commit run --all-files
• git diff --check upstream/main

[gemini-code-assist]

Code Review

This pull request removes explicit checks for the WANDB_API_KEY environment variable in validate_generator_cfg and WandbTracker, deferring authentication directly to the wandb library. Unit tests have been added to ensure that the configuration validation and tracker initialization succeed even when the environment variable is not set. I have no feedback to provide as the changes are clean and well-tested.