Paged attention unit tests

[Aditya Jadhav (aditjadh)]

Adds an automated test suite for Paged Attention that validates:

Test Coverage

1. Correctness across varying prompt lengths and KV block boundaries
2. Output consistency across different runtime configurations
3. Batch invariance guarantees
4. Prefix caching functionality in multi-chat scenarios
5. Prefix caching performance improvements for long-context workloads
6. Model accuracy with and without prefix caching enabled
7. Multimodal (vision-language) support with Paged Attention

Models Covered
Text Models : meta-llama/Llama-3.1-8B-Instruct
Multimodal Models : Qwen/Qwen2.5-VL-3B-Instruct