Fix symbol lookup misidentifying class constants

[PopClom]

My attempt to fix #97988.

The issue was caused by GDScriptLanguage::lookup_code checking base class constants and enums without considering the context. I added a condition to ensure this check only runs when the lookup context is COMPLETION_IDENTIFIER, preventing incorrect symbol resolution in other cases.

[HolonProduction]

Can't fully test at the moment due to #101089 but it would probably work for the specific case from my issue.

However this approach doesn't fully solve the issue:

extends Control

enum Test {
    SIZE_EXPAND,
    SOMETHINGELSE = SIZE_EXPAND, # looking up SIZE_EXPAND here should also resolve to the enum member.
}