Fix UID support for script extends paths

[4d49]

Closes #103479. Now when a string literal is used after extends, it passes through ResourceUID::ensure_path() which:

1. Automatically converts UID strings (uid://...) to their mapped resource paths
2. Maintains compatibility with traditional path strings

The error handling was improved to only set extends_path for valid string literals, rejecting non-string types with appropriate error messages. This maintains type safety while expanding functionality. But I'm not sure about that...

While this PR fixes the issue by enabling UID support for script inheritance paths, I'm not entirely confident in my implementation approach. Given my past experience where I accidentally broke the engine's script debugging mode... I'd appreciate if maintainers could pay extra attention during review.

[vnen]

IMO this should be resolved in the analyzer. The parser only needs to care that it has a valid syntax, which it already does.

In the analyzer, the resolve_class_inheritance() method is the place where this actually gets sorted out figuring out where the path leads.