test: add null parameter validation tests

[alxkm]

This PR adds null parameter validation tests for two components to ensure proper error handling at API boundaries.

Changes

AzureOpenAiRuntimeHints:

• Added test verifying that registerHints() throws NullPointerException when RuntimeHints parameter is null
• Ensures the method validates required parameters before processing

ResourceCacheService:

• Added test verifying that getCachedResource() throws IllegalArgumentException when URI is null
• Validates that error message contains "Location must not be null" for clear debugging
• Prevents NullPointerException from bubbling up to callers

[ilayaperumalg]

@alxkm Thanks for the PR! and @sunyuhan1998 Thanks for the review! Rebased and merged as c03a5ca