Context

Unify provider filter parameter naming between frontend and API. The findings endpoint now accepts both provider and provider_id parameters, allowing the frontend to use provider_id__in consistently across all pages without translation.

Description

• Add provider_id and provider_id__in filter aliases to CommonFindingFilters in the API, enabling both parameter names to work on findings endpoints
• Update all frontend components to use provider_id__in consistently when navigating to findings page (risk-plot, sankey-chart, severity-over-time, status-chart, watchlist, attack-surface, threat-map)
• Remove mapProviderFiltersForFindings helper functions from lib/provider-helpers.ts - no longer needed since API accepts both parameters
• Simplify AccountsSelector and ProviderTypeSelector by removing dual filter key handling

Steps to review

1. API backward compatibility: Verify api/src/backend/api/filters.py:95-102 - both provider and provider_id params point to the same field (scan__provider__id)
2. Frontend consistency: Confirm all navigation to findings uses provider_id__in:
   - risk-plot-client.tsx:48
   - finding-severity-over-time.tsx:69
   - sankey-chart.tsx (removed mapProviderFiltersForFindings calls)
3. Removed helper functions: Verify lib/provider-helpers.ts no longer exports mapProviderFiltersForFindings or mapProviderFiltersForFindingsObject
4. Test navigation: Click through from overview charts (Risk Plot, Sankey, Status Chart, Watchlist) to findings page - verify provider filters are preserved and results are correctly filtered
5. Type safety: Run pnpm run typecheck to ensure no type errors from removed imports

Checklist

• Are there new checks included in this PR? Yes / No
  • If so, do we need to update permissions for the provider? Please review this carefully.
• Review if the code is being covered by tests.
• Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.
• Review if is needed to change the Readme.md
• Ensure new entries are added to CHANGELOG.md, if applicable.

UI

• All issue/task requirements work as expected on the UI
• Screenshots/Video of the functionality flow (if applicable) - Mobile (X < 640px)
• Screenshots/Video of the functionality flow (if applicable) - Table (640px > X < 1024px)
• Screenshots/Video of the functionality flow (if applicable) - Desktop (X > 1024px)
• Ensure new entries are added to CHANGELOG.md, if applicable.

API

• Verify if API specs need to be regenerated.
• Check if version updates are required (e.g., specs, Poetry, etc.).
• Ensure new entries are added to CHANGELOG.md, if applicable.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.