[3.0] Compare URL schemes case insensitively

[albertlast]

Note

This change was produced by an LLM. The code, the commit message and this
description were all written by Claude (Anthropic), driven by @albertlast. It has
not yet had human code review.

Everything stated below was verified by actually running it, rather than only
reasoned about. Even so, please review it as untrusted work: the diagnosis may be
right while the fix is not what SMF would prefer stylistically or architecturally.

Description

RFC 3986, section 3.1: "scheme names are case-insensitive". SMF\Url keeps the scheme
exactly as it was written rather than normalizing it, and isScheme() compares the two
with in_array():

return !empty($this->scheme) && \in_array($this->scheme, array_map('strval', (array) $scheme));

So a URL written with an uppercase scheme does not match its own name.

Two callers depend on this. isWebsite() is isScheme(['http', 'https']), so
HTTP://example.com is not recognised as a website. And User uses it to decide
whether an avatar value is an inline image or a remote address:

$member->avatar->url->isScheme('data')

which means a DATA: URI is treated as a remote URL.

This folds both sides before comparing. The comparison is also made strict while it is
there, since in_array() without the third argument was comparing loosely.

How this was verified

PHP 8.4.23. Same script before and after:

URL	isScheme('https')	isWebsite()
https://example.com	true → true	true → true
HTTPS://example.com	false → true	false → true
HtTp://example.com	false → false	false → true
ftp://example.com	false → false	false → false

URL	isScheme('data')
data:image/png;base64,…	true → true
DATA:image/png;base64,…	false → true

Lowercase schemes, which is nearly everything in practice, behave exactly as before.

php -l and PHP-CS-Fixer clean.

Relationship to other PRs

Sources/Url.php is not touched by any other open PR.

Found while writing unit tests for the stateless parts of the codebase (#9326), which
carries the regression tests for this fix. This PR is independent of that one and can
be merged on its own.

Issues References (Fixes|Related|Closes)

1. No existing issue found for this.
2. Related: [3.0] Add a PHPUnit suite for the parts that need no database #9326