Refactor Policy Enforcer to Policy Checker

- Renamed the `PolicyEnforcer` interface and related implementations to `PolicyChecker` for clarity and consistency.
- Updated configuration keys in YAML files to reflect the new `checkPolicy` terminology.
- Adjusted related code, tests, and documentation to support the new naming convention and ensure compatibility.
- Enhanced comments and examples for the `checkPolicy` configuration to improve usability.

pkg/plugin/implementation/opapolicychecker/testdata/example.rego

@@ -0,0 +1,52 @@
+package policy
+
+import rego.v1
+
+# Example policy: validation rules for beckn messages.
+# This demonstrates the structured result format used with policy_query_path.
+#
+# Available inputs:
+#   - input: the full JSON message body (beckn request)
+#   - data.config: runtime config from the adapter YAML (e.g., minDeliveryLeadHours)
+
+# Default result: valid with no violations.
+default result := {
+  "valid": true,
+  "violations": []
+}
+
+# Compute the result from collected violations.
+result := {
+  "valid": count(violations) == 0,
+  "violations": violations
+}
+
+# Require provider details on confirm
+violations contains "confirm: missing provider in order" if {
+    input.context.action == "confirm"
+    not input.message.order.provider
+}
+
+# Require at least one fulfillment on confirm
+violations contains "confirm: order has no fulfillments" if {
+    input.context.action == "confirm"
+    not input.message.order.fulfillments
+}
+
+# Require billing details on confirm
+violations contains "confirm: missing billing info" if {
+    input.context.action == "confirm"
+    not input.message.order.billing
+}
+
+# Require payment details on confirm
+violations contains "confirm: missing payment info" if {
+    input.context.action == "confirm"
+    not input.message.order.payment
+}
+
+# Require search intent
+violations contains "search: missing intent" if {
+    input.context.action == "search"
+    not input.message.intent
+}