feat(xdebug): switch Xdebug off in the PHP worker

Composer restarts itself with Xdebug unloaded because Xdebug makes PHP
several times slower. Xdebug is never loaded into this process, so what
needs dealing with is the PHP worker: it is spawned with
`-d xdebug.mode=off` and `XDEBUG_MODE=off` (Xdebug reads the environment
variable first and lets it override every ini setting), which makes its
module init return before it installs any executor, compile, error or
opcode hook.

Rewriting the ini files and re-executing, the way xdebug-handler does,
would additionally cover Xdebug 2, which hooks unconditionally and has no
equivalent setting. That is not worth its machinery here: Xdebug 2 caps
out at PHP 7.4, while every PHP version Composer supports can run
Xdebug 3.

What remains of XdebugHandler is small enough to live beside the worker
it governs, so its crate is gone and its callers inline it. isXdebugActive
answers false without asking PHP whenever the worker is switched off, so a
command that needs no PHP does not spawn one just for the Xdebug warning;
diagnose reports what the worker measures instead, which still surfaces an
Xdebug that ignores the setting. PlatformRepository has no unloaded
extension to restore, since switching the mode off leaves it loaded.

COMPOSER_ORIGINAL_INIS is neither written nor read: it exists so a
restarted process can name the ini files it replaced, and IniHelper can
report the worker's own. IniHelperTest injects through that variable, so
none of its cases are ported.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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