chore: import upstream snapshot with attribution

dotnet/test/VectorData/PgVector.ConformanceTests/README.md

@@ -0,0 +1,64 @@
+# PostgreSQL Vector Store Conformance Tests
+
+This project contains conformance tests for the PostgreSQL (pgvector) Vector Store implementation.
+
+## Running the Tests
+
+By default, the tests will automatically use a testcontainer to spin up a PostgreSQL instance with the pgvector extension. Docker must be running on your machine for this to work.
+
+### Using an External PostgreSQL Instance
+
+If you want to run the tests against an external PostgreSQL instance (e.g., a cloud database, local PostgreSQL server, or any other instance), you can provide a connection string through one of the following methods:
+
+**Note**: The external PostgreSQL instance must have the `pgvector` extension available. The tests will attempt to create the extension if it doesn't exist.
+
+#### Option 1: Environment Variable
+
+Set the `Postgres__ConnectionString` environment variable:
+
+```bash
+# Bash/Linux/macOS
+export Postgres__ConnectionString="Host=myserver.postgres.database.azure.com;Database=mydb;Username=myuser;Password=mypassword;"
+
+# PowerShell
+$env:Postgres__ConnectionString = "Host=myserver.postgres.database.azure.com;Database=mydb;Username=myuser;Password=mypassword;"
+```
+
+#### Option 2: Configuration File
+
+Create a `testsettings.development.json` file in this directory with the following content:
+
+```json
+{
+  "Postgres": {
+    "ConnectionString": "Host=myserver.postgres.database.azure.com;Database=mydb;Username=myuser;Password=mypassword;"
+  }
+}
+```
+
+This file is git-ignored and safe for local development.
+
+#### Option 3: User Secrets
+
+```bash
+cd test/VectorData/PgVector.ConformanceTests
+dotnet user-secrets set "Postgres:ConnectionString" "Host=myserver.postgres.database.azure.com;Database=mydb;Username=myuser;Password=mypassword;"
+```
+
+## Benefits of Using an External Instance
+
+Using an external PostgreSQL instance can be beneficial when:
+- You want to avoid the overhead of spinning up Docker containers
+- You need to test against a specific PostgreSQL version or configuration
+- You want faster test execution (no container startup time)
+- You're running tests in an environment where Docker is not available
+- You need to test against cloud-hosted PostgreSQL (e.g., Azure Database for PostgreSQL, AWS RDS)
+
+## Prerequisites for External Instances
+
+The external PostgreSQL instance must:
+- Have the `pgvector` extension installed and available
+- Have sufficient permissions for the connecting user to:
+  - Create the vector extension (if not already created)
+  - Create tables and indexes
+  - Insert, update, and delete data