[GIT-PULLS] [php-src] PR #23433: ext/readline: declare true as the return type of readline_completion_function()

Pull Request: https://github.com/php/php-src/pull/23433
Author: lacatoire

`readline_completion_function()` is declared `bool`, but the function assigns a static function pointer to `rl_attempted_completion_function` and then returns `RETURN_BOOL(rl_attempted_completion_function != NULL)` — a tautology. The only other exit is the `TypeError` from ZPP. `true` is always returned.

The commit declares `true` and returns it directly instead of the redundant pointer comparison.

`ksort()`, `asort()`, and `natsort()` already declare `true` in ext/standard, so this follows existing practice.

Changes: `readline.stub.php` (`bool` → `true`), `readline.c` (`RETURN_BOOL(...)` → `RETURN_TRUE`), regenerated arginfo, added test.