fix(setup): normalize canonical webroot consistently

[joshtrichards]

• Resolves: #

Summary

Prevent double-slash artifacts caused when non-CLI returned '/' for root. And make the underlying implementation consistent.

• Ensure non‑CLI branch returns the canonical webroot ('' for root installs, '/subpath' for subdirectory installs)
• Extract CLI parsing into helper
• Centralize normalization to avoid duplicated/conflicting logic

For ease of review (since I also did a refactor to streamline things), the problematic lines were 534 & 536 in master. CLI mode always made sure to drop the trailing slash and thus handled a root installation fine by returning '' (correct behavior):

$webRoot = rtrim((parse_url($webRoot, PHP_URL_PATH) ?? ''), '/');

But the non-CLI mode code path would return '/' (inconsistent with CLI mode and also incorrect) for the same root installation:

$webRoot = !empty(\OC::$WEBROOT) ? \OC::$WEBROOT : '/';

Often not a serious problem, but was technically incorrect nonetheless.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)