fix(claude): accept both `max_tokens` and `max_completion_tokens` in OpenAI→Claude request conversion

- Prefer `max_tokens` when both `max_tokens` and `max_completion_tokens` are present, otherwise use whichever exists.
- Default to existing template `max_tokens` limit when neither field is provided.
- Add regression coverage for all token-limit source/preference paths.

Closes: #5040