[2.x] fix: Detect the web memory limit from Debian/Ubuntu's FPM conf.d - #4991

Changes proposed in this pull request:

flarum info reports the web SAPI's memory_limit by scanning a fixed list of config files and one hard-coded conf.d directory (/usr/local/etc/php/conf.d, the Docker layout). On Debian and Ubuntu the effective value is usually set in /etc/php/{version}/fpm/conf.d/ — which nothing in the list covered — so the detection fell through to the base php.ini value and reported a figure well below the real one (a report in the wild: "Web: 128M" shown against a real 512M).

That output is exactly what people paste into support threads, so a wrong figure actively misleads diagnosis.

• The conf.d scan now covers the Debian/Ubuntu FPM directories (/etc/php/{version}/fpm/conf.d and the /etc/php{version}/... variant) alongside the Docker one.
• Within a conf.d directory the last matching file wins, mirroring how PHP loads conf.d in name order (a later 99-*.ini overrides an earlier 10-*.ini), rather than returning the first match found.
• The file list and the conf.d list are now their own protected methods, so the detection can be tested against fixture directories instead of the real system paths.

This is a display-only diagnostic; it changes nothing about how Flarum runs.

Reviewers should focus on:

• That the default conf.d list includes the Debian/Ubuntu FPM path — there's a test pinning exactly that, since it's the bug.
• That "last match wins" within a directory is the right ordering for conf.d.

Necessity

• Has the problem that is being solved here been clearly explained? — flarum info misreports the web memory limit on the most common distro layout.
• If applicable, have various options for solving this problem been considered? — scanning the distro conf.d is the direct fix; parsing php-fpm -i output would be heavier and not always available.
• For core PRs, does this need to be in core, or could it be in an extension? — flarum info is a core command.
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. — no frontend changes.
• Frontend changes: tests are green — n/a.
• Frontend changes: tests have been added — n/a.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here. — tests cover reading a limit from a conf.d override, later-file precedence, direct-file precedence over conf.d, the empty case, and that the default list covers the Debian/Ubuntu FPM path.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). — no database involvement.
• The description above is written by me and describes what this pull request actually does.

Required changes:

• Related documentation PR: (Remove if irrelevant)