Python: Refactoring and fix bug. AuthorRole instead of string literals. (#6391)

### Motivation and Context

<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->

There's an enum called `AuthorRole`. I replaced string literals with the
enum values. PyCharm highlights this in some cases: Expected type
'AuthorRole', got 'str' instead

I also fixed a bug in _function_invocation_filters_stream.py_ where the
argument name was wrong. `author="assistant"` instead of
`role="assistant"`

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Evan Mattson <[email protected]>
Co-authored-by: Eduard van Valkenburg <[email protected]>

python/semantic_kernel/connectors/ai/ollama/services/ollama_chat_completion.py

@@ -12,6 +12,7 @@ from semantic_kernel.connectors.ai.chat_completion_client_base import ChatComple
 from semantic_kernel.connectors.ai.ollama.ollama_prompt_execution_settings import OllamaChatPromptExecutionSettings
 from semantic_kernel.connectors.ai.ollama.utils import AsyncSession
 from semantic_kernel.connectors.ai.text_completion_client_base import TextCompletionClientBase
+from semantic_kernel.contents import AuthorRole
 from semantic_kernel.contents.chat_history import ChatHistory
 from semantic_kernel.contents.chat_message_content import ChatMessageContent
 from semantic_kernel.contents.streaming_chat_message_content import StreamingChatMessageContent
@@ -66,7 +67,7 @@ class OllamaChatCompletion(TextCompletionClientBase, ChatCompletionClientBase):
                 ChatMessageContent(
                     inner_content=response_object,
                     ai_model_id=self.ai_model_id,
-                    role="assistant",
+                    role=AuthorRole.ASSISTANT,
                     content=response_object.get("message", {"content": None}).get("content", None),
                 )
             ]
@@ -105,7 +106,7 @@ class OllamaChatCompletion(TextCompletionClientBase, ChatCompletionClientBase):
                     break
                 yield [
                     StreamingChatMessageContent(
-                        role="assistant",
+                        role=AuthorRole.ASSISTANT,
                         choice_index=0,
                         inner_content=body,
                         ai_model_id=self.ai_model_id,
@@ -131,7 +132,7 @@ class OllamaChatCompletion(TextCompletionClientBase, ChatCompletionClientBase):
         """
         if not settings.ai_model_id:
             settings.ai_model_id = self.ai_model_id
-        settings.messages = [{"role": "user", "content": prompt}]
+        settings.messages = [{"role": AuthorRole.USER, "content": prompt}]
         settings.stream = False
         async with (
             AsyncSession(self.session) as session,
@@ -165,7 +166,7 @@ class OllamaChatCompletion(TextCompletionClientBase, ChatCompletionClientBase):
         """
         if not settings.ai_model_id:
             settings.ai_model_id = self.ai_model_id
-        settings.messages = [{"role": "user", "content": prompt}]
+        settings.messages = [{"role": AuthorRole.USER, "content": prompt}]
         settings.stream = True
         async with (
             AsyncSession(self.session) as session,

python/semantic_kernel/connectors/ai/open_ai/services/open_ai_chat_completion_base.py

@@ -249,12 +249,12 @@ class OpenAIChatCompletionBase(OpenAIHandler, ChatCompletionClientBase):
 
     def _chat_message_content_to_dict(self, message: "ChatMessageContent") -> dict[str, str | None]:
         msg = super()._chat_message_content_to_dict(message)
-        if message.role == "assistant":
+        if message.role == AuthorRole.ASSISTANT:
             if tool_calls := getattr(message, "tool_calls", None):
                 msg["tool_calls"] = [tool_call.model_dump() for tool_call in tool_calls]
             if function_call := getattr(message, "function_call", None):
                 msg["function_call"] = function_call.model_dump_json()
-        if message.role == "tool":
+        if message.role == AuthorRole.TOOL:
             if tool_call_id := getattr(message, "tool_call_id", None):
                 msg["tool_call_id"] = tool_call_id
             if message.metadata and "function" in message.metadata:

python/semantic_kernel/contents/function_result_content.py

@@ -6,6 +6,7 @@ from xml.etree.ElementTree import Element  # nosec
 
 from pydantic import field_validator
 
+from semantic_kernel.contents.author_role import AuthorRole
 from semantic_kernel.contents.const import FUNCTION_RESULT_CONTENT_TAG, TEXT_CONTENT_TAG
 from semantic_kernel.contents.kernel_content import KernelContent
 from semantic_kernel.contents.text_content import TextContent
@@ -104,8 +105,8 @@ class FunctionResultContent(KernelContent):
         from semantic_kernel.contents.chat_message_content import ChatMessageContent
 
         if unwrap:
-            return ChatMessageContent(role="tool", items=[self.result])  # type: ignore
-        return ChatMessageContent(role="tool", items=[self])  # type: ignore
+            return ChatMessageContent(role=AuthorRole.TOOL, items=[self.result])  # type: ignore
+        return ChatMessageContent(role=AuthorRole.TOOL, items=[self])  # type: ignore
 
     def to_dict(self) -> dict[str, str]:
         """Convert the instance to a dictionary."""