Add minimum coverage threshold (3/6): GraphQL mutations & fields

What does this MR do and why?

This is MR 3 of 6 in a stacked series that adds a minimum coverage threshold to the Coverage-Check approval rule.

MR	Branch	Base	Content
1	6284-coverage-threshold-1-db-model-ff	master	DB migration & model validation (!226363 (merged))
2	6284-coverage-threshold-2-service-rest	6284-coverage-threshold-1-db-model-ff	Service logic, REST API & feature flag (!226370 (merged))
3 (this MR)	6284-coverage-threshold-3-graphql	6284-coverage-threshold-2-service-rest	GraphQL mutations & fields
4	6284-coverage-threshold-4-blockage-reason	6284-coverage-threshold-3-graphql	blockage_reason feature & GraphQL exposure
5	6284-coverage-threshold-5-frontend	6284-coverage-threshold-4-blockage-reason	Frontend: Settings UI + MR widget feedback
6	6284-coverage-threshold-6-docs	master	Documentation (standalone, no dependency)

Splitting was discussed and agreed upon with @kherman1 and @hfyngvason in !224343 (closed).

This MR includes:

• coverage_minimum_threshold argument on approvalProjectRuleUpdate mutation (allows updating the threshold via GraphQL)
• coverage_minimum_threshold argument on branchRuleApprovalProjectRuleCreate mutation (allows setting the threshold when creating a rule)
• coverageMinimumThreshold field on ApprovalProjectRuleType (allows reading the threshold)
• Updated generated GraphQL reference documentation and introspection schemas
• Standardized descriptions to comply with GitLab GraphQL style guide (no demonstrative "this", line length limit)

Files to review in this MR

Note: This is a stacked MR. The diff shows cumulative changes against master. Please focus your review on the files listed below, which are the ones introduced in this MR.

• ee/app/graphql/mutations/approval_project_rules/update.rb - Update mutation argument
• ee/app/graphql/mutations/branch_rules/approval_project_rules/create.rb - Create mutation argument
• ee/app/graphql/types/branch_rules/approval_project_rule_type.rb - Type field
• doc/api/graphql/reference/_index.md - Generated GraphQL docs
• public/-/graphql/introspection_result.json - Generated introspection schema
• public/-/graphql/introspection_result_no_deprecated.json - Generated introspection schema

Related to #6284 Extracted from !224343 (closed)

How to set up and validate locally

Prerequisites

• MR 1 (!226363 (merged)) and MR 2 (!226370 (merged)) must be applied first
• Feature flag must be enabled: Feature.enable(:coverage_minimum_threshold) in Rails console

1. Create a rule with threshold via GraphQL

It is easiest to test via the approvalProjectRuleUpdate mutation. First, create a rule via REST API and get its global ID:

# Create a rule via REST API
curl -s -X POST "http://gdk.local:3000/api/v4/projects/<PROJECT_ID>/approval_rules" \
  -H "PRIVATE-TOKEN: <TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{"name": "Coverage-Check", "approvals_required": 1, "report_type": "code_coverage"}'

Note the id from the response (e.g. 123). The global ID is gid://gitlab/ApprovalProjectRule/123.

Then update it via GraphQL to set the threshold:

curl -s -X POST "http://gdk.local:3000/api/graphql" \
  -H "Authorization: Bearer <TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{
    "query": "mutation { approvalProjectRuleUpdate(input: { id: \"gid://gitlab/ApprovalProjectRule/<RULE_ID>\", name: \"Coverage-Check\", approvalsRequired: 1, coverageMinimumThreshold: 80.0 }) { approvalRule { id name coverageMinimumThreshold } errors } }"
  }'

Expected response:

{
  "data": {
    "approvalProjectRuleUpdate": {
      "approvalRule": {
        "id": "gid://gitlab/ApprovalProjectRule/123",
        "name": "Coverage-Check",
        "coverageMinimumThreshold": 80.0
      },
      "errors": []
    }
  }
}

2. Query the threshold

curl -s -X POST "http://gdk.local:3000/api/graphql" \
  -H "Authorization: Bearer <TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{
    "query": "query { project(fullPath: \"<PROJECT_PATH>\") { branchRules { nodes { approvalRules { nodes { id name coverageMinimumThreshold } } } } } }"
  }'

3. Run the specs

bundle exec rspec ee/spec/graphql/types/branch_rules/approval_project_rule_type_spec.rb

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

• The change is backwards compatible. The new argument is optional, existing queries are unaffected.
• GraphQL changes follow the style guide (no demonstrative "this", line length respected).
• Mutations use existing CreateService/UpdateService, no new service logic introduced.

Related to #6284