test: convert Bruno tests from .bru to .yml format

Phase 12 - Bruno API Tests Standardization

Convert all custom section builder tests to .yml format:
- preview-collection.bru → preview-collection.yml
- create-custom-section-manual.bru → create-custom-section-manual.yml
- create-custom-section-missing-fields.bru → create-custom-section-missing-fields.yml
- create-custom-section-rules.bru → create-custom-section-rules.yml

All Bruno tests now consistently use .yml extension across the project.
Tests cover:
- Preview collection with filter rules
- Create collection with manual book selection
- Validation of required fields
- Multiple filter rules with operators

Part of Carousel Dashboard Plan completion

bruno/collections/create-custom-section-missing-fields.yml

@@ -0,0 +1,23 @@
+meta:
+  name: Create Custom Section Missing Required Fields
+  type: http
+  seq: 3
+http:
+  method: POST
+  url: '{{base_url}}/api/collections'
+  auth: inherit
+  body:
+    type: json
+    jsonBody: "{\n  \"library_id\": \"{{library_id}}\",\n  \"icon\": \"📚\"\
+      \n}"
+runtime:
+  scripts:
+  - type: tests
+    code: "test(\"status must be 400 with missing name\", function() {\n    expect(res.status).to.eql(400);\n\
+      });\n\ntest(\"response contains error message\", function() {\n    expect(res.body.error).to.exist;\n\
+      \  });"
+
+docs: |-
+  Test error handling when creating a collection without required fields.
+
+  **Expected Response**: 400 Bad Request with error message.