[GIT-PULLS] [php-src] PR #23402: ext/standard: Change return type of header_register_callback() to true

Pull Request: https://github.com/php/php-src/pull/23402
Author: lacatoire

`header_register_callback()` is declared as returning `bool`, but `PHP_FUNCTION(header_register_callback)` in `main/SAPI.c` has a single non-throwing exit, `RETURN_TRUE`.

The `false` return is a leftover from PHP 7:

```c
/* php-7.0.0, main/SAPI.c */
if (zend_parse_parameters(ZEND_NUM_ARGS(), "z", &callback_func) == FAILURE) {
    return;
}
if (!zend_is_callable(callback_func, 0, NULL)) {
    RETURN_FALSE;
}
```

PHP 8.0.0 moved the check to the `f` parameter specifier, which throws a `TypeError`, and the `RETURN_FALSE` went away with it. The declared type was never adjusted.

Checked on 8.4.22 and 8.5.8: every accepted callable form (closure, function name, `Class::method`, `[$obj, 'method']`, `__invoke` object, first-class callable syntax) returns `true`, an invalid callback throws `TypeError`, and registering after `headers_sent()` is already `true` — the case where the callback is knowingly discarded — returns `true` as well.

`true` being a subtype of `bool`, the only observable change is what Reflection reports.

Whether the `headers_sent` branch ought to report failure instead is a behaviour question, deliberately left out of this change.