fix: emit nnUNet store_true flags as bare CLI args in nnUNetV2Runner (#8968)

Fixes #8237 .

### Description
The runner passed boolean flags like `--c` through as `--c True`, but
nnU-Net treats these as `store_true` flags that take no value, so it
errored out with `unrecognized arguments: True`. Now we emit just the
bare flag when it's set, drop it when it isn't, and leave the other args
alone.

### Types of changes
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved how command-line options are passed for training and
validation, so boolean settings now behave as expected.
* Validation now runs through the standard training workflow with
validation enabled, helping ensure more consistent results.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Soumya Snigdha Kundu <soumya_snigdha.kundu@kcl.ac.uk>
Signed-off-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: aymuos15

monai/apps/nnunet/nnunetv2_runner.py

@@ -596,16 +596,13 @@ def train_single_model_command(
         if self.export_validation_probabilities:
             cmd.append("--npz")
 
-        store_true_flags = {"c", "val", "use_compressed", "disable_checkpointing"}
         for _key, _value in kwargs.items():
-            if _key in store_true_flags:
+            prefix = "-" if _key in {"p", "pretrained_weights"} else "--"
+            if isinstance(_value, bool):
                 if _value:
-                    cmd.append(f"--{_key}")
-            elif _key in {"p", "pretrained_weights"}:
-                if _value:
-                    cmd += [f"-{_key}", str(_value)]
+                    cmd.append(f"{prefix}{_key}")
             else:
-                cmd += [f"--{_key}", str(_value)]
+                cmd += [f"{prefix}{_key}", str(_value)]
 
         cmd_str: list[str] = [str(c) for c in cmd]