feat(integrations): add merge-method selection and provider-specific payloads

Introduce dedicated merge handling for integration review merges:

- Add src-tauri/src/integrations/merge.rs: implements merge_options (read provider repo settings), merge_payload (build provider-specific merge body) and a Tauri command get_integration_review_merge_options. Includes unit tests for behavior.
- Wire merge module into integrations.rs and pass an optional merge_method into provider-specific review action functions (GitHub, GitLab, Gitea, Azure DevOps). run_integration_review_action now accepts an optional merge_method, validates it early, and includes provider-specific merge payloads when performing a merge.
- Export the new command in src-tauri/src/main.rs so the frontend can request merge options.

Frontend changes to support selecting a merge method before merging:

- ConfirmDialog.svelte: add SelectMenu support and a select field to confirm requests.
- ReviewCenter.svelte: fetch integration merge options, show a merge-method selector in the merge confirmation, and pass the chosen method to the review action.
- Update types and git bindings to surface IntegrationMergeOptions / IntegrationMergeMethod and the getIntegrationReviewMergeOptions call (git.ts / types.ts changes staged).

Effect: users can pick a merge method appropriate to the provider/project; the integration layer generates the correct API payload per provider. Tests added for merge logic.

src-tauri/src/main.rs

@@ -38,7 +38,7 @@ use integrations::{
     create_integration_review_request, list_integration_repository_branches,
     add_integration_review_comment, get_integration_review_details, list_integration_repositories, list_integration_review_requests, open_in_browser,
     create_integration_issue, list_azure_issue_projects, list_azure_issue_types,
-    run_integration_review_action, list_integration_issues, get_integration_board, list_integration_boards, move_integration_board_card, list_integration_issue_comments, add_integration_issue_comment, close_integration_issue, list_azure_issue_states, set_azure_issue_state,
+    run_integration_review_action, get_integration_review_merge_options, list_integration_issues, get_integration_board, list_integration_boards, move_integration_board_card, list_integration_issue_comments, add_integration_issue_comment, close_integration_issue, list_azure_issue_states, set_azure_issue_state,
 };
 use std::path::{Path, PathBuf};
 use std::sync::Mutex;
@@ -463,7 +463,7 @@ async fn main() {
             set_azure_issue_state,
             get_integration_review_details,
             add_integration_review_comment,
-            run_integration_review_action,
+            run_integration_review_action, get_integration_review_merge_options,
             open_in_browser,
             set_sync_badge,
             close_splashscreen,