[CLI] predict(ckpt_path="best") raises ValueError even though ModelCheckpoint is configured under trainer.callbacks

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### Bug description

When running trainer.predict via LightningCLI with --ckpt_path best, Lightning raises:
```bash
ValueError: `.predict(ckpt_path="best")` is set but `ModelCheckpoint` is not configured to save the best model.
```

However, ModelCheckpoint is configured under trainer.callbacks in the YAML, and training is executed via LightningCLI. This looks like either:
1. LightningCLI isn’t detecting the ModelCheckpoint from the CLI YAML when resolving best during predict, or
2. The checkpoint_connector’s detection logic for best doesn’t handle the CLI/YAML path or log directory resolution, or
3. A mismatch between monitor metric registration and best-checkpoint discovery is not surfaced clearly (i.e., the callback exists, but the condition for “best” was silently unmet).

### What version are you seeing the problem on?

v2.5

### Reproduced in studio

_No response_

### How to reproduce the bug

```python
trainer:
callbacks:
- class_path: lightning.pytorch.callbacks.ModelCheckpoint
init_args:
monitor: val_loss
mode: min
save_top_k: 1
filename: "{val_loss:.4f}"
save_last: false

uv run src/cli.py predict -c config.yaml --ckpt_path=best
```

### Error messages and logs

```
# Error messages and logs here please
```

### Environment

Current environment

```
#- PyTorch Lightning Version (e.g., 2.5.0):
#- PyTorch Version (e.g., 2.5):
#- Python version (e.g., 3.12):
#- OS (e.g., Linux):
#- CUDA/cuDNN version:
#- GPU models and configuration:
#- How you installed Lightning(`conda`, `pip`, source):
```

### More info

_No response_

cc @lantiga @mauvilsa

Start with src/cli.py and the LightningCLI predict entry point, then trace how --ckpt_path=best reaches the checkpoint_connector. Reproduce with the provided config.yaml and compare trainer.callbacks configuration with best-checkpoint discovery. Done means the supplied ModelCheckpoint configuration is correctly handled, or the failure explains the unmet condition clearly.

Written by the indexing model from the issue text.