Incorrect error message for an invalid usage of the walrus operator in a nested comprehension

**Describe the bug**
Incorrect error message for an invalid usage of the walrus operator in a nested comprehension.

**Code or Screenshots**
If possible, provide a minimal, self-contained code sample (surrounded by triple back ticks) to demonstrate the issue. The code should define or import all referenced symbols.

```python
print([
f"{i=}"
for i in [
x
for j in range(10)
if (x := j * 2) % 3 == 0
]
])
```

Pylance gives:

> Operator ":=" is not allowed in this context without surrounding parentheses

Note the expression is already surrounded by parentheses.

Whereas running the code gives:

> SyntaxError: assignment expression cannot be used in a comprehension iterable expression

**VS Code extension or command-line**

VS Code:
```
Client: Pylance client (2026.2.1) started with python extension (2026.4.0)
Pylance language server 2026.2.1 (pyright version 1.1.408, commit 138238b4)
```

Start by running the supplied nested-comprehension sample through Pyright and compare its diagnostic with Python's SyntaxError. Trace the diagnostic path for the walrus operator and comprehension iterable, then verify that the reported error matches the valid surrounding parentheses and the actual invalid usage.

Written by the indexing model from the issue text.