Rename backend risk classification strings to risk assessment

What does this MR do and why?

The Product team decided to call the feature "Risk Assessment" instead of "Risk Classification". This MR updates the backend translatable strings to match the new name. The frontend strings, in the merge request widget, change in !257523 (merged). Later MRs will rename component names and the underlying namespaces to match the product name and avoid confusion.

This MR changes text only:

• For backend strings, the gettext namespace RiskClassification| becomes RiskAssessment|
• The foundational flow display name changes from "Risk Classification" to "Risk Assessment"
• GraphQL descriptions for MergeRequestRiskAssessment, MergeRequest.riskAssessment, and the MergeRequestRiskAssessmentStatus enum values now say "assessment" instead of "classification". Type and field names do not change, so there is no schema change for clients.
• One API error message changes: "workflow_id X is not a classification session for this merge request." becomes "workflow_id X is not a risk assessment session for this merge request."

References

• Tracking issue: #630589
• Frontend strings: !257523 (merged)

Screenshots or screen recordings

No visual change is expected in the merge request widget, because the English text stays the same. The flow name change is visible in the group GitLab Duo settings.

Before	After

How to set up and validate locally

1. In a Rails console, enable the flags: Feature.enable(:duo_mr_risk_classification) and Feature.enable(:show_duo_mr_risk_classification_widget).

2. Go to the gitlab-duo group GitLab Duo configuration page (e.g. http://gdk.test:3000/groups/gitlab-duo/-/edit#js-gitlab-duo-settings). Find the list of foundational flows and confirm the flow shows as "Risk Assessment" with the description "Assess the risk of a merge request so review effort can be routed to the changes that warrant it."

3. Create a merge request in gitlab-duo/test. Confirm the risk assessment widget renders as before (for example "Risk assessment pending", then the tier and signal breakdown once the flow completes).

4. In GraphQL Explorer (http://gdk.test:3000/-/graphql-explorer), run this query and confirm the descriptions say "assessment":

   {
     __type(name: "MergeRequestRiskAssessmentStatus") {
       description
       enumValues { name description }
     }
   }

MR acceptance checklist

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