refactor: remove dead inference API code and clean up imports (#4093)

# What does this PR do?

Delete ~2,000 lines of dead code from the old bespoke inference API that
was replaced by OpenAI-only API. This includes removing unused type
conversion functions, dead provider methods, and event_logger.py.

Clean up imports across the codebase to remove references to deleted
types. This eliminates unnecessary
code and dependencies, helping isolate the API package as a
self-contained module.

This is the last interdependency between the .api package and "exterior"
packages, meaning that now every other package in llama stack imports
the API, not the other way around.

## Test Plan

this is a structural change, no tests needed.

---------

Signed-off-by: Charlie Doern <cdoern@redhat.com>

tests/unit/providers/utils/inference/test_prompt_adapter.py

@@ -0,0 +1,35 @@
+# 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 llama_stack.apis.inference import (
+    OpenAIAssistantMessageParam,
+    OpenAIUserMessageParam,
+)
+from llama_stack.models.llama.datatypes import RawTextItem
+from llama_stack.providers.utils.inference.prompt_adapter import (
+    convert_openai_message_to_raw_message,
+)
+
+
+class TestConvertOpenAIMessageToRawMessage:
+    """Test conversion of OpenAI message types to RawMessage format."""
+
+    async def test_user_message_conversion(self):
+        msg = OpenAIUserMessageParam(role="user", content="Hello world")
+        raw_msg = await convert_openai_message_to_raw_message(msg)
+
+        assert raw_msg.role == "user"
+        assert isinstance(raw_msg.content, RawTextItem)
+        assert raw_msg.content.text == "Hello world"
+
+    async def test_assistant_message_conversion(self):
+        msg = OpenAIAssistantMessageParam(role="assistant", content="Hi there!")
+        raw_msg = await convert_openai_message_to_raw_message(msg)
+
+        assert raw_msg.role == "assistant"
+        assert isinstance(raw_msg.content, RawTextItem)
+        assert raw_msg.content.text == "Hi there!"
+        assert raw_msg.tool_calls == []