Fix Tinker training for ServiceClient SDK

[Sudhendra]

Summary

• add ServiceClient-mode training path in train_on_tinker when legacy tinker.Client is unavailable
• pass API key via ServiceClient(api_key=...) to match current SDK requirements and avoid session init failure
• add regression coverage for ServiceClient-only environments in tests/test_train_tinker.py

Test Plan

• pytest tests/test_train_tinker.py tests/test_tinker_sdk_client.py tests/test_tinker_data.py -q
• ruff check src/training/train_tinker.py tests/test_train_tinker.py

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 5bae6eaf4a

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".