Honor cache controls for GenAI runtime compilation

Problem

eval and perf --runtime winml-genai can compile pre-built GenAI bundles into a persistent _compiled/ directory, but the shared model cache controls do not govern that runtime compilation cache.

• --rebuild cannot currently force regeneration of _compiled/.
• --no-use-cache cannot currently compile into a disposable directory without touching the bundle cache.
• The behavior differs from the model-artifact cache controlled by the same flags.

Scope

• Define cache/rebuild semantics for GenaiSession runtime compilation.
• Thread the resolved policy through WinMLGenaiCausalLM, winml eval, and winml perf --runtime winml-genai.
• Ensure --rebuild replaces persistent compiled artifacts.
• Ensure --no-use-cache uses disposable compiled artifacts and leaves _compiled/ untouched.
• Add focused tests for cache reuse, forced rebuild, temporary compilation, and pre-built bundles.
• Keep model build caching and runtime compilation caching clearly distinguished in warnings and documentation.

Start by tracing cache policy through GenaiSession and WinMLGenaiCausalLM, then follow how winml eval and winml perf --runtime winml-genai invoke runtime compilation. Add focused tests for cache reuse, forced rebuild, temporary compilation, and pre-built bundles. Done means --rebuild replaces persistent artifacts, --no-use-cache leaves _compiled/ untouched, and warnings and documentation distinguish the two caches.