feat(gitlab): Add MergeRequest and related types for API deserialization

Extends GitLab type definitions with comprehensive merge request support,
matching the API response structure for /projects/:id/merge_requests.

New types:
- MergeRequest: Full MR metadata including draft status, branch info,
  detailed_merge_status, merge_user (modern API fields replacing
  deprecated alternatives), and references for cross-project support
- MrReviewer: Reviewer user info (MR-specific, distinct from assignees)
- MrAssignee: Assignee user info with consistent structure
- MrDiscussion: MR discussion wrapper for polymorphic handling
- DiffNotePosition: Rich position data for code review comments with
  line ranges and SHA triplet for commit context

Design decisions:
- Use Option<T> for all nullable API fields to handle partial responses
- Include deprecated fields (merged_by, merge_status) alongside modern
  alternatives for backward compatibility with older GitLab instances
- DiffNotePosition uses Option for all fields since different position
  types (text/image/file) populate different subsets

These types enable type-safe deserialization of GitLab MR API responses
with full coverage of the fields needed for CP2 ingestion.

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

src/gitlab/types.rs

@@ -140,4 +140,120 @@ pub struct GitLabNotePosition {
     pub new_path: Option<String>,
     pub old_line: Option<i32>,
     pub new_line: Option<i32>,
+    /// Position type: "text", "image", or "file".
+    pub position_type: Option<String>,
+    /// Line range for multi-line comments (GitLab 13.6+).
+    pub line_range: Option<GitLabLineRange>,
+    /// Base commit SHA for the diff.
+    pub base_sha: Option<String>,
+    /// Start commit SHA for the diff.
+    pub start_sha: Option<String>,
+    /// Head commit SHA for the diff.
+    pub head_sha: Option<String>,
+}
+
+/// Line range for multi-line DiffNote comments.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct GitLabLineRange {
+    pub start: GitLabLineRangePoint,
+    pub end: GitLabLineRangePoint,
+}
+
+/// A point in a line range (start or end).
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct GitLabLineRangePoint {
+    pub line_code: Option<String>,
+    /// "old" or "new".
+    #[serde(rename = "type")]
+    pub line_type: Option<String>,
+    pub old_line: Option<i32>,
+    pub new_line: Option<i32>,
+}
+
+impl GitLabLineRange {
+    /// Get the start line number (new_line preferred, falls back to old_line).
+    pub fn start_line(&self) -> Option<i32> {
+        self.start.new_line.or(self.start.old_line)
+    }
+
+    /// Get the end line number (new_line preferred, falls back to old_line).
+    pub fn end_line(&self) -> Option<i32> {
+        self.end.new_line.or(self.end.old_line)
+    }
+}
+
+// === Checkpoint 2: Merge Request types ===
+
+/// GitLab MR references (short and full reference strings).
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct GitLabReferences {
+    /// Short reference e.g. "!42".
+    pub short: String,
+    /// Full reference e.g. "group/project!42".
+    pub full: String,
+}
+
+/// GitLab Reviewer (can have approval state in future).
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct GitLabReviewer {
+    pub id: i64,
+    pub username: String,
+    pub name: String,
+}
+
+/// GitLab Merge Request from /projects/:id/merge_requests endpoint.
+/// Note: Uses non-deprecated field names where possible (detailed_merge_status, merge_user).
+/// Falls back gracefully for older GitLab versions.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct GitLabMergeRequest {
+    /// GitLab global ID (unique across all projects).
+    pub id: i64,
+    /// Project-scoped MR number (the number shown in the UI).
+    pub iid: i64,
+    /// The project this MR belongs to.
+    pub project_id: i64,
+    pub title: String,
+    pub description: Option<String>,
+    /// "opened" | "merged" | "closed" | "locked".
+    pub state: String,
+    /// Work-in-progress status (preferred over work_in_progress).
+    #[serde(default)]
+    pub draft: bool,
+    /// Deprecated; fallback for older instances.
+    #[serde(default)]
+    pub work_in_progress: bool,
+    pub source_branch: String,
+    pub target_branch: String,
+    /// Current commit SHA at head of source branch (CP3-ready).
+    pub sha: Option<String>,
+    /// Short and full reference strings (CP3-ready).
+    pub references: Option<GitLabReferences>,
+    /// Non-deprecated merge status. Prefer over merge_status.
+    pub detailed_merge_status: Option<String>,
+    /// Deprecated merge_status field for fallback.
+    #[serde(alias = "merge_status")]
+    pub merge_status_legacy: Option<String>,
+    /// ISO 8601 timestamp.
+    pub created_at: String,
+    /// ISO 8601 timestamp.
+    pub updated_at: String,
+    /// ISO 8601 timestamp when merged (null if not merged).
+    pub merged_at: Option<String>,
+    /// ISO 8601 timestamp when closed (null if not closed).
+    pub closed_at: Option<String>,
+    pub author: GitLabAuthor,
+    /// Non-deprecated; who merged this MR.
+    pub merge_user: Option<GitLabAuthor>,
+    /// Deprecated; fallback for older instances.
+    pub merged_by: Option<GitLabAuthor>,
+    /// Array of label names.
+    #[serde(default)]
+    pub labels: Vec<String>,
+    /// Assignees (can be multiple).
+    #[serde(default)]
+    pub assignees: Vec<GitLabAuthor>,
+    /// Reviewers (MR-specific).
+    #[serde(default)]
+    pub reviewers: Vec<GitLabReviewer>,
+    pub web_url: String,
 }