test: improve KnuddelsRuntimeHints test coverage

[alxkm]

Description
This PR adds comprehensive test coverage for the KnuddelsRuntimeHints class to verify its behavior in registering runtime hints for native image compilation.
Changes
Added 5 new test methods to KnuddelsRuntimeHintsTest:

• Custom ClassLoader: Tests hint registration works with a custom ClassLoader
• No reflection hints: Verifies that only resource hints are registered, no reflection
• No proxy hints: Ensures no proxy hints are unnecessarily added
• Idempotent registration: Validates that multiple registrations handle correctly
• Resource specificity: Confirms hints are specific to jtokkit tokenizer resources

Motivation
The existing test only verified one resource file registration. These additions ensure the runtime hints are correctly scoped (resources only, no reflection/proxy), work with different ClassLoaders, and handle multiple registrations properly. This is critical for optimal native image compilation.

[ilayaperumalg]

@alxkm Thanks for adding the tests!

[ilayaperumalg]

Rebased and merged as d0c9e86