refactor: fix psalm 6 issues

[roxblnfk]

Q	A
Bugfix?	❌
Breaks BC?	❌
New feature?	❌
Issues	—
Docs PR	—

Psalm 6 (errorLevel=1) is now green. Every issue is fixed in code; nothing is suppressed except ClassMustBeFinal.

• #[\Override] on every method that implements an interface. PHP 8.2 ignores the attribute, so no polyfill.
• !empty() checks on ?non-empty-string are now strict null checks (Installed, VersionChecker). The result of preg_match() is checked directly.
• Package skips a composer.json it cannot read instead of passing false to json_decode().
• ProcessInterface::exec() documents $command as array<string>, matching what Symfony Process expects.
• ClassMustBeFinal is disabled in psalm.xml, since making public classes final would break BC. The unused UndefinedAttributeClass suppression for JetBrains ExpectedValues is removed.

Review notes

• The VersionChecker::greaterThan() docblock widens from non-empty-string|null to string|null, because the method has always treated '' like null. The one difference from empty(): '0' is now passed through as a version instead of falling back to the required version.

• How was this tested:

  • Psalm passes locally with --php-version=8.2 and 8.3
  • Testo suite passes locally (119/119)

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