fix: Resolve Llama4 tool calling 500 errors (Issue #2584)

This commit fixes the tool calling failures with Llama4 models that were
returning 500 errors while Together API worked correctly. The root cause
was that the system was using Llama3's JSON format for all models instead
of Llama4's python_list format.

Key changes:
- NEW: llama_stack/models/llama/llama4/interface.py - Complete Llama4 interface
  with python_list tool format support
- MODIFIED: prompt_adapter.py - Added model-aware decode_assistant_message()
  that uses Llama4ChatFormat for llama4 models and Llama3ChatFormat for others
- MODIFIED: openai_compat.py - Updated to pass model_id parameter to enable
  model-specific format detection
- MODIFIED: sku_list.py - Enhanced with provider alias support for better
  model resolution
- NEW: tests/unit/models/test_decode_assistant_message.py - Comprehensive unit
  tests for the new decode_assistant_message function

The fix ensures that:
- Llama4 models (meta-llama/Llama-4-*) use python_list format: [func(args)]
- Other models continue using JSON format: {"type": "function", ...}
- Backward compatibility is maintained for existing models
- Tool calling works correctly across different model families
- Graceful fallback when Llama4 dependencies are unavailable

Testing:
- All 17 unit tests pass (9 original + 8 new)
- Conditional imports prevent torch dependency issues
- Comprehensive test coverage for different model types and scenarios

Fixes #2584

llama_stack/models/llama/sku_list.py

@@ -22,6 +22,20 @@ def resolve_model(descriptor: str) -> Model | None:
     for m in all_registered_models():
         if descriptor in (m.descriptor(), m.huggingface_repo):
             return m
+
+    # Check provider aliases by attempting to import and check common providers
+    try:
+        from llama_stack.providers.remote.inference.together.models import MODEL_ENTRIES as TOGETHER_ENTRIES
+
+        for entry in TOGETHER_ENTRIES:
+            if descriptor in entry.aliases and entry.llama_model:
+                # Find the model by its descriptor
+                for m in all_registered_models():
+                    if m.descriptor() == entry.llama_model:
+                        return m
+    except ImportError:
+        pass
+
     return None