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/preview-collection.yml

@@ -0,0 +1,60 @@
+meta:
+  name: Preview Collection
+  type: http
+  seq: 1
+http:
+  method: POST
+  url: '{{base_url}}/api/collections/preview'
+  auth: inherit
+  body:
+    type: json
+    jsonBody: "{\n  \"library_id\": \"{{library_id}}\",\n  \"rules\": [\n\
+      {\n      \"id\": \"rule1\",\n      \"field\": \"genre\",\n\
+      \"operator\": \"equals\",\n      \"value\": \"Fiction\",\n\
+      \"priority\": 1\n    }\n  ],\n  \"manual_book_ids\": [],\n  \"limit\": 20\n}"
+runtime:
+  scripts:
+  - type: tests
+    code: "test(\"status must be 200 with valid request\", function() {\n    expect(res.status).to.eql(200);\n\
+      });\n\ntest(\"response contains items array\", function() {\n    expect(res.body.items).to.exist;\n\
+         expect(res.body.items).to.be.an('array');\n  });"
+
+docs: |-
+  Preview collection with filter rules before saving.
+
+  **Endpoint**: POST /api/collections/preview
+  **Auth**: Required (Bearer token)
+
+  ## Request Body
+
+  | Field | Type | Required | Description |
+  |-------|------|----------|-------------|
+  | library_id | string | Yes | Library UUID |
+  | rules | array | No | Filter rules to evaluate |
+  | manual_book_ids | array | No | Manually selected book IDs |
+  | limit | int | No | Max items to return (default: 20, max: 100) |
+
+  ## Example Request
+
+  ```json
+  {
+    "library_id": "cc23c3a7-f8fb-451a-a78d-2a16df1b725a",
+    "rules": [
+      {
+        "id": "rule1",
+        "field": "genre",
+        "operator": "equals",
+        "value": "Fiction",
+        "priority": 1
+      }
+    ],
+    "manual_book_ids": [],
+    "limit": 20
+  }
+  ```
+
+  ## Response
+
+  Returns array of matching book items.
+
+  This endpoint is used by the Custom Section Builder to show users what books will match their filter rules before they save the collection.