Handle root-namespace classes in SymfonyFileLocator

[Junaid-PK]

Fixes #520.

When a mapping lookup fails for a class in the root namespace, strrpos() returns false. The assertion then fails when assertions are enabled; when they are disabled, the subsequent substr() starts at offset 1 and reports a filename such as ateTime.yml.

Use the complete class name when there is no namespace separator, while preserving the existing short-name behavior for namespaced classes. The regression test verifies that DateTime produces the expected MappingException and filename.

Validation:

• vendor/bin/phpunit — 146 tests, 306 assertions
• vendor/bin/phpcs — 72 files
• vendor/bin/phpstan analyse --no-progress --memory-limit=512M — no errors