chore(rename): move llama_stack.distribution to llama_stack.core (#2975)

We would like to rename the term `template` to `distribution`. To
prepare for that, this is a precursor.

cc @leseb

llama_stack/core/utils/context.py

@@ -0,0 +1,40 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from collections.abc import AsyncGenerator
+from contextvars import ContextVar
+
+
+def preserve_contexts_async_generator[T](
+    gen: AsyncGenerator[T, None], context_vars: list[ContextVar]
+) -> AsyncGenerator[T, None]:
+    """
+    Wraps an async generator to preserve context variables across iterations.
+    This is needed because we start a new asyncio event loop for each streaming request,
+    and we need to preserve the context across the event loop boundary.
+    """
+    # Capture initial context values
+    initial_context_values = {context_var.name: context_var.get() for context_var in context_vars}
+
+    async def wrapper() -> AsyncGenerator[T, None]:
+        while True:
+            try:
+                # Restore context values before any await
+                for context_var in context_vars:
+                    context_var.set(initial_context_values[context_var.name])
+
+                item = await gen.__anext__()
+
+                # Update our tracked values with any changes made during this iteration
+                for context_var in context_vars:
+                    initial_context_values[context_var.name] = context_var.get()
+
+                yield item
+
+            except StopAsyncIteration:
+                break
+
+    return wrapper()