docs: update Bruno terminology to OpenCollection YAML format

Update all references from "Bruno DSL .bru files" to "Bruno OpenCollection YAML .yml files" to reflect the current Bruno format. This includes:

- PROJECT_GUIDELINES.md: Update API testing requirements
- README.md: Update command examples
- TEST_DATA.md: Update test data references
- docs/contributing/development.md: Update API testing section
- docs/developer/api-reference.md: Update Bruno testing documentation
- docs/developer/collections-api.md: Update test file references
- scripts/README.md: Update validation script documentation
- scripts/verify-guidelines.sh: Update file extension check (.bru → .yml)
- bruno/opencollection.yml: Rename collection from "Untitled Collection" to "Bookhoard"

bruno/opencollection.yml

@@ -1,7 +1,7 @@
 opencollection: 1.0.0
 
 info:
-  name: Untitled Collection
+  name: Bookhoard
 config:
   proxy:
     inherit: true
@@ -21,8 +21,8 @@ request:
 
 docs:
   content: |-
-    # Bruno API Tests for Bookhoard
-    This directory contains Bruno collection for testing the Bookhoard API with comprehensive REST documentation.
+    # Bruno OpenCollection YAML API Tests for Bookhoard
+    This directory contains Bruno OpenCollection YAML format tests for testing the Bookhoard API with comprehensive REST documentation.
      Setup
     1. Install Bruno: https://www.usebruno.com/
     2. Open Bruno and import this collection folder