Indicate max_concurrency for throughput and disallow running standalone without --rate (#467)

## Summary

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Add a indicator to throughput similar to concurrent's `concurrent@CONC`.
Additionally require specifying `--rate` for throughput when running
standalone rather than defaulting to `MAX_CONCURRENCY`.

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: sjmonson

src/guidellm/benchmark/profiles.py

@@ -432,6 +432,10 @@ def resolve_args(
         kwargs.pop("random_seed", None)
         if rate is not None and len(rate) > 0:
             kwargs["max_concurrency"] = rate[0]
+        else:
+            # Require explicit max_concurrency; in the future max_concurrency
+            # should be dynamic and rate can specify some tunable
+            raise ValueError("ThroughputProfile requires a rate parameter")
         return kwargs
 
     @property

src/guidellm/scheduler/strategies.py

@@ -387,7 +387,7 @@ def __str__(self) -> str:
         """
         :return: String identifier for throughput strategy
         """
-        return "throughput"
+        return f"throughput@{self.max_concurrency or 'unlimited'}"
 
     @property
     def processes_limit(self) -> PositiveInt | None:

tests/unit/scheduler/test_strategies.py

@@ -335,9 +335,10 @@ def test_string_representation(
         self, valid_instances: tuple[ThroughputStrategy, dict]
     ):
         """Test __str__ method for ThroughputStrategy."""
-        instance, _ = valid_instances
+        instance, constructor_args = valid_instances
+        max_concurrency = constructor_args.get("max_concurrency")
         result = str(instance)
-        assert result == "throughput"
+        assert result == f"throughput@{max_concurrency or 'unlimited'}"
 
     @pytest.mark.smoke
     def test_marshalling(self, valid_instances: tuple[ThroughputStrategy, dict]):