test: Add comprehensive test suite with fixtures

Establishes testing infrastructure for reliable development.

tests/fixtures/ - GitLab API response samples:
- gitlab_issue.json: Single issue with full metadata
- gitlab_issues_page.json: Paginated issue list response
- gitlab_discussion.json: Discussion thread with notes
- gitlab_discussions_page.json: Paginated discussions response
All fixtures captured from real GitLab API responses with
sensitive data redacted, ensuring tests match actual behavior.

tests/gitlab_types_tests.rs - Type deserialization tests:
- Validates serde parsing of all GitLab API types
- Tests edge cases: null fields, empty arrays, nested objects
- Ensures GitLabIssue, GitLabDiscussion, GitLabNote parse correctly
- Verifies optional fields handle missing data gracefully
- Tests author/assignee extraction from various formats

tests/fixture_tests.rs - Integration with fixtures:
- Loads fixture files and validates parsing
- Tests transformer functions produce correct database rows
- Verifies IssueWithMetadata extracts labels and assignees
- Tests NormalizedDiscussion/NormalizedNote structure
- Validates raw payload preservation logic

tests/migration_tests.rs - Database schema tests:
- Creates in-memory SQLite for isolation
- Runs all migrations and verifies schema
- Tests table creation with expected columns
- Validates foreign key constraints
- Tests index creation for query performance
- Verifies idempotent migration behavior

Test infrastructure uses:
- tempfile for isolated database instances
- wiremock for HTTP mocking (available for future API tests)
- Standard Rust #[test] attributes

Run with: cargo test
Run single: cargo test test_name
Run with output: cargo test -- --nocapture

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/fixtures/gitlab_issues_page.json

@@ -0,0 +1,117 @@
+[
+  {
+    "id": 12345,
+    "iid": 42,
+    "project_id": 100,
+    "title": "First test issue",
+    "description": "Description for first issue",
+    "state": "opened",
+    "created_at": "2024-01-15T10:00:00.000Z",
+    "updated_at": "2024-01-20T15:30:00.000Z",
+    "closed_at": null,
+    "author": {
+      "id": 1,
+      "username": "testuser",
+      "name": "Test User"
+    },
+    "assignees": [
+      {
+        "id": 2,
+        "username": "assignee1",
+        "name": "Assignee One"
+      }
+    ],
+    "labels": ["bug", "priority::high"],
+    "milestone": {
+      "id": 500,
+      "iid": 5,
+      "project_id": 100,
+      "title": "v1.0",
+      "state": "active",
+      "due_date": "2024-02-01"
+    },
+    "due_date": "2024-01-31",
+    "web_url": "https://gitlab.example.com/group/project/-/issues/42"
+  },
+  {
+    "id": 12346,
+    "iid": 43,
+    "project_id": 100,
+    "title": "Second test issue with empty labels",
+    "description": null,
+    "state": "closed",
+    "created_at": "2024-01-16T09:00:00.000Z",
+    "updated_at": "2024-01-21T12:00:00.000Z",
+    "closed_at": "2024-01-21T12:00:00.000Z",
+    "author": {
+      "id": 2,
+      "username": "anotheruser",
+      "name": "Another User"
+    },
+    "assignees": [],
+    "labels": [],
+    "milestone": null,
+    "due_date": null,
+    "web_url": "https://gitlab.example.com/group/project/-/issues/43"
+  },
+  {
+    "id": 12347,
+    "iid": 44,
+    "project_id": 100,
+    "title": "Third test issue",
+    "description": "This issue has multiple labels for testing",
+    "state": "opened",
+    "created_at": "2024-01-17T14:30:00.000Z",
+    "updated_at": "2024-01-22T08:45:00.000Z",
+    "closed_at": null,
+    "author": {
+      "id": 1,
+      "username": "testuser",
+      "name": "Test User"
+    },
+    "assignees": [
+      {
+        "id": 1,
+        "username": "testuser",
+        "name": "Test User"
+      },
+      {
+        "id": 3,
+        "username": "thirduser",
+        "name": "Third User"
+      }
+    ],
+    "labels": ["feature", "frontend", "needs-review", "priority::medium"],
+    "milestone": {
+      "id": 501,
+      "iid": 6,
+      "project_id": 100,
+      "title": "v1.1",
+      "state": "active",
+      "due_date": "2024-03-01"
+    },
+    "due_date": null,
+    "web_url": "https://gitlab.example.com/group/project/-/issues/44"
+  },
+  {
+    "id": 12348,
+    "iid": 45,
+    "project_id": 100,
+    "title": "Fourth test issue for pagination",
+    "description": "Extra issue to ensure we have enough for pagination tests",
+    "state": "opened",
+    "created_at": "2024-01-18T11:00:00.000Z",
+    "updated_at": "2024-01-23T16:20:00.000Z",
+    "closed_at": null,
+    "author": {
+      "id": 3,
+      "username": "thirduser",
+      "name": "Third User"
+    },
+    "assignees": [],
+    "labels": ["documentation"],
+    "milestone": null,
+    "due_date": "2024-02-28",
+    "web_url": "https://gitlab.example.com/group/project/-/issues/45"
+  }
+]