Change additional context to accept string/symbol keys

What does this MR do and why?

Builds on gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1194 (merged) and addresses this comment: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1194 (comment 2045322731)

This MR addresses an issue where the content key in additional_context is sometimes passed as a string and sometimes a symbol. Using indifferent access fixes the issue.

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