[clang-tidy] False positive readability-trailing-comma with brace initialization

Example:

```cpp
void f()
{
int x{123 +
456};
}
```

Leads to:
```
source>:4:14: warning: initializer list should have a trailing comma [readability-trailing-comma]
4 | 456};
| ^
| ,
1 warning generated.
```

[Godbolt.](https://godbolt.org/z/n9T6ovheq)

Start by reproducing the warning from the C++ example and Godbolt link, then trace the clang-tidy readability-trailing-comma check named in the report. Add coverage for brace initialization whose expression spans lines, and verify that this false positive no longer appears while the check still reports genuine missing trailing commas.

Written by the indexing model from the issue text.