fix(config): seed no-model refs for prepare-data

[rodboev]

Summary

ng_prepare_data still fails when the no-model parse path encounters additional response-model refs such as user_model or judge_model, because only policy_model gets a dummy server config.

This seeds missing referenced responses_api_models server blocks during the no-model parse path and leaves the existing dummy cleanup to handle populated real model configs.

Closes #997

Root cause

GlobalConfigDictParserConfig.NO_MODEL_GLOBAL_CONFIG_DICT only provides a dummy policy_model. After the parser merges configs, server-ref validation checks referenced model-server names against top-level server instances. When a config references responses_api_models/user_model and no top-level user_model exists, parsing raises ServerRefNotFoundError.

The existing dummy_model cleanup only rewrites model-server blocks that already exist. It cannot synthesize the missing top-level user_model or judge_model block.

Changes

• Seed missing referenced responses_api_models server blocks only when the no-model parse path is active.
• Reuse the existing dummy model shape and existing cleanup pass.
• Add focused parser coverage for arbitrary non-policy model-server refs.
• Add a direct prepare_data() entry-path regression test.

Scope

• No schema redesign.
• No benchmark-specific branches.
• No changes to resources-server or agent-server refs.
• No dependency, docs, or provider changes.

Validation

• uv run pytest tests/unit_tests/test_global_config.py tests/unit_tests/test_train_data_utils.py -k "dummy_model or no_model_config_seeds_referenced_model_server_refs or prepare_data_no_model_config_seeds_non_policy_model_refs" -x -v - blocked before collection on Windows because uvloop==0.21.0 does not support Windows
• uv run pre-commit run --files nemo_gym/global_config.py tests/unit_tests/test_global_config.py tests/unit_tests/test_train_data_utils.py - not run because uv run is blocked by the same Windows uvloop build limitation
• ruff check --config pyproject.toml nemo_gym/global_config.py tests/unit_tests/test_global_config.py tests/unit_tests/test_train_data_utils.py
• ruff format --config pyproject.toml --check nemo_gym/global_config.py tests/unit_tests/test_global_config.py tests/unit_tests/test_train_data_utils.py
• Broader CI matrix - covered by pull_request workflows, not run locally

Notes

The new regression should fail on the current base with ServerRefNotFoundError for responses_api_models/'user_model', then pass after the parser seeds the missing dummy model-server block.

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