Define safer behavior when encore_entry_*_source files are missing

[javihgil]

Problem

encore_entry_css_source() and encore_entry_js_source() read asset files directly from disk with file_get_contents().

If the built files are missing, moved, or the configured public path is wrong, the current behavior is not explicit and can degrade badly at runtime.

Current evidence

• src/Twig/Extension/EncoreEntrySourcesExtension.php concatenates file contents directly from the resolved file paths
• there is no explicit handling for unreadable or missing files

Expected behavior

The bundle should define and document a clear behavior for missing asset files.

Suggested work

• decide whether the helper should throw a clear exception or return an empty string
• add tests covering missing files and wrong public paths
• document the chosen behavior in FEATURES.md and armonic-docs