feat(core): add multi-strategy ERS to support ldap and sql (#2596)

### Proposed Changes

Add a multi-strategy adapter that will support a variety of backends
(LDAP, SQL, etc) but support a chaining approach to satisfy the needs
for entity chains.

Also added contract focused integration tests to ensure all ERS adapters
behave the same given the same input.

### Checklist

- [ ] I have added or updated unit tests
- [ ] I have added or updated integration tests (if appropriate)
- [ ] I have added or updated documentation

### Testing Instructions

Author: jrschumacher

.github/workflows/pr-checks.yaml

@@ -60,6 +60,7 @@ jobs:
             docs
             sdk
             policy
+            ers
             kas
             authz
             examples

README.md

@@ -9,6 +9,7 @@
 ## Documentation
 
 - [Configuration](./docs/Configuring.md)
+- [Multi-Strategy Entity Resolution Service](./ERS_TESTING.md)
 - [Development](#for-contributors)
 - [Policy Config Schema](./service/policy/db/schema_erd.md)
 - [Policy Config Testing Diagram](./service/integration/testing_diagram.png)
@@ -112,6 +113,45 @@ go run ./service start
 ```
 <!-- END copy ./service/README#quick-start -->
 
+### Multi-Strategy Entity Resolution Service
+
+The OpenTDF platform supports a powerful multi-strategy Entity Resolution Service (ERS) that can integrate with multiple identity providers and data sources simultaneously.
+
+#### Quick Start with Multi-Strategy ERS (Preview)
+
+> **⚠️ Preview Feature**: Multi-Strategy ERS is in preview (V2 only). APIs may change.
+
+To run OpenTDF with comprehensive entity resolution using SQL and LDAP providers:
+
+```sh
+# Start core infrastructure + ERS test services
+docker compose --profile ers-test up
+
+# Use the multi-strategy configuration
+go run ./service start --config opentdf-ers-test.yaml
+```
+
+This enables entity resolution from:
+- **JWT Claims** - Direct token claim extraction
+- **PostgreSQL** - SQL database queries for organizational data
+- **LDAP/Active Directory** - Directory service integration
+
+#### ERS Provider Testing
+
+Test the multi-strategy ERS functionality:
+
+```sh
+# Run integration tests (Docker services provide the backends automatically)
+go test ./service/entityresolution/integration -run TestMultiStrategy -v
+```
+
+#### Configuration Options
+
+- **`opentdf-ers-test.yaml`** - Complete OpenTDF platform with multi-strategy ERS
+- **`ERS_TESTING.md`** - Comprehensive documentation and examples
+
+The multi-strategy ERS (preview) provides enterprise-grade identity resolution with failover, multiple provider support, and flexible mapping strategies.
+
 ## For Contributors
 
 This section is focused on the development of the OpenTDF platform.