test: Add comprehensive test suite for MR ingestion

Introduces thorough test coverage for merge request functionality,
following the established testing patterns from issue ingestion.

New test files:
- mr_transformer_tests.rs: NormalizedMergeRequest transformation tests
  covering full MR with all fields, minimal MR, draft detection via
  title prefix and work_in_progress field, label/assignee/reviewer
  extraction, and timestamp conversion

- mr_discussion_tests.rs: MR discussion normalization tests including
  polymorphic noteable binding, DiffNote position extraction with
  line ranges and SHA triplet, and resolvable note handling

- diffnote_position_tests.rs: Exhaustive DiffNote position scenarios
  covering text/image/file types, single-line vs multi-line comments,
  added/removed/modified lines, and missing position handling

New fixtures:
- fixtures/gitlab_merge_request.json: Representative MR API response
  with nested structures for integration testing

Updated tests:
- gitlab_types_tests.rs: Add MR type deserialization tests
- migration_tests.rs: Update expected schema version to 6

Test design follows property-based patterns where feasible, with
explicit edge case coverage for nullable fields and API variants
across different GitLab versions.

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

tests/fixtures/gitlab_merge_request.json

@@ -0,0 +1,27 @@
+{
+  "id": 12345,
+  "iid": 42,
+  "project_id": 100,
+  "title": "Add user authentication",
+  "description": "Implements JWT auth flow",
+  "state": "merged",
+  "draft": false,
+  "work_in_progress": false,
+  "source_branch": "feature/auth",
+  "target_branch": "main",
+  "sha": "abc123def456",
+  "references": { "short": "!42", "full": "group/project!42" },
+  "detailed_merge_status": "mergeable",
+  "merge_status": "can_be_merged",
+  "created_at": "2024-01-15T10:00:00Z",
+  "updated_at": "2024-01-20T14:30:00Z",
+  "merged_at": "2024-01-20T14:30:00Z",
+  "closed_at": null,
+  "author": { "id": 1, "username": "johndoe", "name": "John Doe" },
+  "merge_user": { "id": 2, "username": "janedoe", "name": "Jane Doe" },
+  "merged_by": { "id": 2, "username": "janedoe", "name": "Jane Doe" },
+  "labels": ["enhancement", "auth"],
+  "assignees": [{ "id": 3, "username": "bob", "name": "Bob Smith" }],
+  "reviewers": [{ "id": 4, "username": "alice", "name": "Alice Wong" }],
+  "web_url": "https://gitlab.example.com/group/project/-/merge_requests/42"
+}