feat: resolve spark routes filters for custom placeholders

[paulbalandan]

Description
php spark routes printed <unknown> in the Before/After Filters columns for any route using a custom placeholder registered with $routes->addPlaceholder(). SampleURIGenerator had no sample for such placeholders and substituted
::unknown::, producing a URI that matched no route, so FilterCollector could not resolve its filters. Filter execution at request time was unaffected. This was only the command's display, as confirmed in the issue.

This adds two ways to obtain a sample URI for a custom placeholder:

• PlaceholderSampleGenerator derives a value from the placeholder's regular expression, covering the fragments common in route patterns (character classes, shorthand escapes, quantifiers, literals, top-level alternation, anchors). Patterns it cannot reverse (lookarounds, backreferences, named groups, etc.) return null, and every candidate is validated with preg_match before use.
• Config\Routing::$placeholderSamples lets you set or override the sample per placeholder. A value that does not match the placeholder regex is ignored.

SampleURIGenerator resolves each placeholder in this order: a matching$placeholderSamples value, the built-in sample, an auto-generated value, then ::unknown::. Results are memoized per placeholder.

Common patterns such as [A-Z]{3}[0-9]+ and UUIDs now resolve without any configuration. The user guide, changelog, and upgrade notes are updated.

Fixes #9804
Supersedes and closes #10206

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Looks good, thanks!

Bottom line... That's pretty compex for something that's supposed to handle such a small feature ;)