readability-trailing-comma - false positive + default function argument

LLVM version 23.1.1

`clang-tidy --checks='-*,readability-trailing-comma' test.cpp -- -std=c++26`

Reproducer:

```c++
struct Options
{
int option1{};
int option2{};
};

class Class
{
public:
static void setOptions(Options opts1 = {}, Options opts2 = {})
{
(void)opts1;
(void)opts2;
}

private:
};

```

Output:

```
test.cpp:11:46: error: initializer list should not have a trailing comma [readability-trailing-comma,-warnings-as-errors]
11 | static void setOptions(Options opts1 = {}, Options opts2 = {})
| ^
```

Start by running the clang-tidy command against the supplied test.cpp reproducer with C++26 enabled, then trace the readability-trailing-comma check that reports the default arguments. Done means the valid empty initializer lists in default function arguments no longer produce a diagnostic, while genuine trailing-comma cases remain covered.

Written by the indexing model from the issue text.