Fix key mismatch and context access in PubMedQA

## Describe the bug
The `pubmed_qa_prompt` function failed with a `KeyError: 'QUESTION'` and `IndexError`. This was due to the function expecting uppercase keys (`QUESTION`, `CONTEXTS`) and a flat context structure, whereas the actual dataset uses lowercase keys (`question`, `context`) and a nested `context['contexts']` structure.
Also, the gold answer in the dataset is in lower case, but the LLM sometimes generates the answer in upper case, and this leads to false negative judgement.

## To Reproduce
```python
task = "pubmedqa|0"

pipeline = Pipeline(
tasks=task,
pipeline_parameters=pipeline_params,
evaluation_tracker=evaluation_tracker,
model_config=model_config,
)

pipeline.evaluate()
pipeline.save_and_push_results()
pipeline.show_results()
```
```python
in Pipeline.__init__(self, tasks, pipeline_parameters, evaluation_tracker, model_config, model, metric_options)
140 # We init tasks first to fail fast if one is badly defined
141 self._init_random_seeds()
--> 142 self._init_tasks_and_requests(tasks=tasks)
144 self.model_config = model_config
145 self.accelerator, self.parallel_context = self._init_parallelism_manager()
...
30 choices=[line["final_decision"]],
31 gold_index=0,
32 )

KeyError: 'QUESTION'
```

## Expected behavior
- The function should correctly access the lowercase keys and path to contexts.
- The metric should be case insensitive exact match

## Version info
- OS: mac
- Lighteval version: main (local development)