Security Full Data Set – Backend (1) – Remove `MAX_FINDINGS_COUNT` cap and introduce feature flag
## TL;DR
Introduce the `security_mr_report_full_finding_set` feature flag and, when enabled, remove the 25-finding cap in `SecurityFindingsReportsComparer` so `added` and `fixed` return the complete undismissed finding set per report type.
## Acceptance Criteria
- [ ] Feature flag exists, defaults to disabled, and uses `project` as the actor.
- [ ] With the flag disabled the current behaviour of `added` and `fixed` methods stay the same.
- [ ] With the flag enabled, `added` and `fixed` methods return every finding in the respective set, with no slicing.
- [ ] Specs cover both flag states.
## Implementation Details
##### Files:
- Newly created: config/feature_flags/gitlab_com_derisk/security_mr_report_full_finding_set.yml`
- `ee/lib/gitlab/ci/reports/security/security_findings_reports_comparer.rb`
- `ee/spec/lib/gitlab/ci/reports/security/security_findings_reports_comparer_spec.rb`
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## Testing Approach
- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual validation steps
## Estimate
issue