The undefined variable goes unnoticed

## Environment data

- Pylance version: 2025.6.2
- OS and version: 6.6.87.2-microsoft-standard-WSL2 (Ubuntu)
- Python version (& distribution if applicable, e.g. Anaconda): Python 3.12.2

## Code Snippet

```python

counter += 1

```

ends up with correct:

"counter" is not defined [PyLance]

while

```python

for i in range(100):
counter += 1

```

PyLance does not report anything

It seems that PyLance does not care about undefined variables in nested loops.

## Expected behavior

The undefined variable in the nested loop is reported

## Actual behavior

The undefined variable goes unnoticed.

Start by reproducing the two snippets in the reported Pylance environment: the top-level increment should produce an undefined-variable diagnostic, while the increment inside the loop currently does not. Trace the undefined-variable analysis for loop bodies and add or update coverage for this case. Done means the nested-loop example reports the undefined variable without regressing the existing behavior.

Written by the indexing model from the issue text.