Bracketed authority ending in @ raises IndexError

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### Describe the bug

The bug is similar to the previously fixed #1485
Malformed authorities containing brackets and ending in `@` raise `IndexError` during `URL()` construction. Malformed URLs normally produce a URL-specific `ValueError`; exposing an internal string subscript error makes validation unnecessarily difficult for callers.

### To Reproduce

```python
from yarl import URL
for value in ("http://[::1]@", "//[]@", "//a[b]c@"):
try:
URL(value)
except Exception as exc:
print(repr(value), type(exc).__name__, str(exc))
```

### Expected behavior

Reject these malformed authorities with `ValueError`, consistent with other invalid bracketed hosts

### Logs/tracebacks

```python-traceback
'http://[::1]@' IndexError string index out of range
'//[]@' IndexError string index out of range
'//a[b]c@' IndexError string index out of range
```

### Python Version

```console
$ python --version
3.12.3
```

### multidict Version

```console
$ python -m pip show multidict
6.7.1
```

### propcache Version

```console
$ python -m pip show propcache
0.4.1
```

### yarl Version

```console
$ python -m pip show yarl
1.24.5, commit `577e75f`
```

### OS

Linux 6.6.87.2, WSL2, x86-64

### Additional context

_No response_