[fsdp] fix: add missing mixed precision configuration to FSDPEngineConfig (volcengine#3068)

### What does this PR do?

The `FSDPEngineConfig` dataclass was missing the `mixed_precision` field
that the runtime code expected. By adding:

```python
mixed_precision: Optional[dict[str, Any]] = None
```

The dataclass now properly supports the mixed precision configuration
that the FSDP workers code uses with `fsdp_config.get("mixed_precision",
None).`


https://github.com/volcengine/verl/blob/55e3c5bc09c85dba09f736eef476f110ad641b75/verl/workers/fsdp_workers.py#L371


Otherwise, if we run with:

```bash
python3 -m verl.trainer.main_ppo \
    actor_rollout_ref.actor.fsdp_config.mixed_precision.param_dtype=bf16 \
    actor_rollout_ref.actor.fsdp_config.mixed_precision.reduce_dtype=fp32 \
    actor_rollout_ref.actor.fsdp_config.mixed_precision.buffer_dtype=fp32 \
    # ... other parameters
```

The following error may occur:

```bash
raise InstantiationException(msg) from e
hydra.errors.InstantiationException: Error in call to target 'verl.workers.config.engine.FSDPEngineConfig':
TypeError("FSDPEngineConfig.__init__() got an unexpected keyword argument 'mixed_precision'")
full_key: actor_rollout_ref.actor.fsdp_config
```

### Backward compatibility

No behavior change for existing configs (default remains None).

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Author: xxrjun

verl/workers/config/engine.py

@@ -92,6 +92,7 @@ class FSDPEngineConfig(BaseConfig):
         forward_prefetch (bool): Whether to prefetch parameters for next forward pass, default False
         model_dtype (str): Model data type used to initialize the transformers model. default "fp32"
         use_orig_params (bool): Whether to use original parameters when initialize FSDP1, default False
+        mixed_precision (Optional[dict[str, Any]]): Mixed precision configuration for FSDP, default None
     """
 
     wrap_policy: dict[str, Any] = field(default_factory=dict)
@@ -103,3 +104,4 @@ class FSDPEngineConfig(BaseConfig):
     forward_prefetch: bool = False
     model_dtype: str = "fp32"
     use_orig_params: bool = False
+    mixed_precision: Optional[dict[str, Any]] = None