fix(backend): filter SDK default credentials from credentials API responses (#12544)

## Summary

- Filter SDK-provisioned default credentials from credentials API list
endpoints
- Reuse `CredentialsMetaResponse` model from internal router in external
API (removes duplicate `CredentialSummary`)
- Add `is_sdk_default()` helper for identifying platform-provisioned
credentials
- Add `provider_matches()` to credential store for consistent provider
filtering
- Add tests for credential filtering behavior

### Changes
- `backend/data/model.py` — add `is_sdk_default()` helper
- `backend/api/features/integrations/router.py` — filter SDK defaults
from list endpoints
- `backend/api/external/v1/integrations.py` — reuse
`CredentialsMetaResponse`, filter SDK defaults
- `backend/integrations/credentials_store.py` — add `provider_matches()`
- `backend/sdk/registry.py` — update credential registration
- `backend/api/features/integrations/router_test.py` — new tests
- `backend/api/features/integrations/conftest.py` — test fixtures

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan

#### Test plan:
- [x] Unit tests for credential filtering (`router_test.py`)
- [x] Verify SDK default credentials excluded from API responses
- [x] Verify user-created credentials still returned normally