AI: LLMChatCompletionMessage always includes value for role attribute

[hannahramadan]

It's possible for a vendor to not provide a role attribute (discovered by .NET in testing Bedrock). The LLMChatCompletionMessage payload must still provide a value for the role attribute, which must be either user (for requests) or assistant (for responses).

Since all AI instrumentation shares some common classes, we should add logic about the strings when the role attribute is missing. Potentially in NewRelic::Agent::Llm::LlmEvent#event_attributes or something in the LlmChatCompletionMessage class.

Spec (internal).

[workato-integration]

https://new-relic.atlassian.net/browse/NR-254343