Fix the following 5 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 5
nemo_retriever/src/nemo_retriever/agentic/retrieval.py:140-146
`num_concurrent` has no validation in `__post_init__`, but `react_max_steps` and `text_truncation` do. A programmatic caller who passes `num_concurrent=0` will not get an error from the config; the failure surfaces later as a `ValueError: max_workers must be greater than 0` from `ThreadPoolExecutor`, with no indication of which config field caused it.

```suggestion
    def __post_init__(self) -> None:
        if not str(self.llm_model).strip():
            raise ValueError("Agentic retrieval requires a non-empty llm_model.")
        if int(self.react_max_steps) < 1:
            raise ValueError("react_max_steps must be >= 1.")
        if int(self.text_truncation) < 0:
            raise ValueError("text_truncation must be >= 0.")
        if int(self.num_concurrent) < 1:
            raise ValueError("num_concurrent must be >= 1.")
```

### Issue 2 of 5
nemo_retriever/src/nemo_retriever/pipeline/__main__.py:641
**Unbounded qrels dict logged at INFO**

`_qrels` is logged in full without any size cap. On a ViDoRe domain with 300+ queries each with multiple relevant docs, this generates a single INFO line that can exceed log-aggregator limits and makes the log unreadable. The `_run` dict immediately below already applies `[:10]` per query; `_qrels` should do the same, or both should be dropped to DEBUG.

### Issue 3 of 5
nemo_retriever/src/nemo_retriever/pipeline/__main__.py:849
**Agentic LLM always uses the general `remote_api_key`**

There is no `--agentic-api-key` CLI flag; the agentic LLM endpoint always receives `remote_api_key`. For deployments where the embedding endpoint and the agentic LLM endpoint are at different services that require different credentials, the wrong key will be sent to the LLM, resulting in an authentication error that gives no hint about which flag to set. Consider adding `--agentic-api-key` (defaulting to `remote_api_key` for backward compatibility) and resolving it the same way `agentic_invoke_url` is resolved from an env var.

### Issue 4 of 5
nemo_retriever/src/nemo_retriever/graph/react_agent_operator.py:534-535
**Per-step INFO logging produces very high log volume**

Each ReAct iteration now emits at least 2-3 INFO lines (`begin seen_docs`, `finish_reason`, and `retrieve` or `final_results`). With `max_steps=50` (the default) and `num_concurrent=N` queries, a single evaluation call produces up to `50 x 3 x N` INFO lines. The step-level loop control logs (`begin`, `finish_reason`, `no tool call; requesting continuation`) are better suited to DEBUG since they carry no actionable information beyond the retrieval and `final_results` calls that already log at INFO.

### Issue 5 of 5
nemo_retriever/src/nemo_retriever/graph/react_agent_operator.py:13-21
**`_preview_text` / `_preview_doc_ids` duplicated in two modules**

Identical implementations of `_preview_text` (and near-identical `_preview_doc_ids`) are defined in both `react_agent_operator.py` and `selection_agent_operator.py` with the same module-level constants (`_LOG_PREVIEW_CHARS = 300`, `_LOG_DOC_ID_LIMIT = 20`). A shared `graph/_utils.py` would keep these in one place and prevent drift if the truncation limit ever needs adjusting.