[6.x] Encode asset URLs

[duncanmcclean]

This pull request fixes an issue where asset URLs weren't encoded, so filenames containing spaces or accented characters produced invalid URLs.

This was happening because Asset::url() and Asset::absoluteUrl() assembled the URL from the raw path, leaving characters like ú and spaces untouched. An asset called Dún Laoghaire_18 2.jpg came back as https://ams3.digitaloceanspaces.com/mywebsite/Dún Laoghaire_18 2.jpg, which Str::isUrl() doesn't consider a URL. Browsers percent-encode a raw src attribute for you, so images still rendered — the breakage only showed up server-side, where passing the URL into something like the Glide tag would send it down the wrong branch.

This PR fixes it by running the path through URL::encode() when assembling both URLs. This PR also decodes the path in AssetRepository::findByUrl(), so encoded URLs still resolve back to their asset.

Fixes #5593

[jasonvarga]

URL::encode() (src/Facades/Endpoint/URL.php:326-346) was built for encoding a URL that may already be partially percent-encoded — it rawurlencode()s and then maps a set of sequences (%2F, %40, %3A, ... %25→%) back to their literal characters so it doesn't double-encode an already-encoded URL. Reusing it here on a raw filesystem path (Asset.php's url()/absoluteUrl()) breaks that assumption: if a filename already contains a literal %XX-looking substring, it gets treated as "already encoded" and passes through untouched.

Repro: an asset with the literal filename photo%20one.jpg:

• Asset::url() encodes it to .../photo%20one.jpg — unchanged, since the existing %20 is preserved as if it were already an encoded space.
• AssetRepository::findByUrl()'s new rawurldecode() then turns that into photo one.jpg, which doesn't match the actual stored path photo%20one.jpg. The asset becomes unresolvable via the url() → findByUrl() round trip.

This is narrow (filenames containing literal percent-encoded-looking sequences), but it's a real regression — these assets resolved fine before this change since neither side encoded/decoded. None of the added test cases (nothing to encode, spaces, accents, spaces in folders) cover a literal %XX substring, so it's not caught by CI.

Needs fixing before merge: don't reuse URL::encode() (designed for full URLs) on raw asset paths. A plain per-segment rawurlencode() — without the "preserve already-encoded" table — would round-trip correctly through rawurldecode() for all inputs, since it never assumes the raw path is pre-encoded.