[DYNAMO] wire TITO client to Dynamo renderer transport

[AmeenP]

Summary

Ports the missing prime-rl client-side TITO wiring from Biswa's tested branches on top of #2420 (feat/dynamo-renderer-transport-config). The shared renderer/verifiers pins and return_token_ids cleanup now live in #2420, so this PR only carries the TITO-specific follow-up.

• treats openai_chat_completions_token as token-aware for client.backend = "dynamo"
• with [DYNAMO] pass renderer transport to verifiers #2420, uses renderer_transport="dynamo" for both renderer-mode and TITO clients when using Dynamo
• adds unit coverage for Dynamo TITO transport selection

Biswa Compatibility Notes

Included from Biswa's rl-sdk-2 where it matches the current Dynamo contract:

• backend-based Dynamo transport selection in this PR
• dropping the global return_token_ids default in [DYNAMO] pass renderer transport to verifiers #2420

Not included yet:

• Biswa's compute_teacher_logprobs dispatch that asks Dynamo chat completions for nvext.extra_fields=["prompt_logprobs"]. Dynamo #9509 currently exposes engine_data / completion token data, not nvext.prompt_logprobs, so copying that prime-rl path would require a matching Dynamo change first.

Validation

Local:

• uv lock --check
• uvx ruff==0.13.0 check --config=pyproject.toml src/prime_rl/utils/client.py tests/unit/utils/test_client.py
• uvx ruff==0.13.0 format --check --config=pyproject.toml src/prime_rl/utils/client.py tests/unit/utils/test_client.py

GitHub Actions:

• Ruff: pass
• Slim install (prime-rl-configs only): pass
• Unit tests: pass

Local test note:

• uv run pytest tests/unit/utils/test_client.py is blocked on macOS by the repo's Linux-only lockfile; uv run --no-sync pytest tests/unit/utils/test_client.py exited 139 locally.