Raise Duo Chat additional context size limits
### Summary GitLab Duo Chat currently has an [additional context size limit](https://gitlab.com/gitlab-org/gitlab/-/blob/10a1b10e2aeda7a59fc07b5ce3d126c53aa86c41/ee/app/graphql/types/ai/additional_context_input_type.rb#L20), where it refers to the [MAX_BODY_SIZE](https://gitlab.com/gitlab-org/gitlab/-/blob/10a1b10e2aeda7a59fc07b5ce3d126c53aa86c41/ee/lib/api/code_suggestions.rb#L12) value of `600_000`. ### Current behavior At present, attempting to upload files into GitLab Duo Chat's context reading that are larger than ~`586KB` in size, will fail and produce the following error message: > “Failed to send the chat message to the API: content is too long (maximum is 600000)” ### What is the desired behavior? The purpose of this issue is to explore if the [MAX_BODY_SIZE](https://gitlab.com/gitlab-org/gitlab/-/blob/10a1b10e2aeda7a59fc07b5ce3d126c53aa86c41/ee/lib/api/code_suggestions.rb#L12) value of `600_000` is possible to raise further. It seems that we may also be setting re-using this value which is set against the code suggestions feature, but different values/thresholds might be more appropriate to set independently across different feature types.
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