feat(integrations): add labels integration and expose shared IntegrationApi

Add a new labels integration (src-tauri/src/integrations/labels.rs) that implements listing,
reading and updating issue labels for multiple providers (GitHub, Gitea, GitLab and Azure
DevOps). The new module provides Tauri commands:
- list_integration_labels
- get_integration_issue_labels
- set_integration_issue_labels

Refactor assignees integration to make core HTTP helpers reusable:
- Rename AssignmentApi -> IntegrationApi and make its fields/methods pub(super).
- Make helper functions api_url, repository_parts and target_url pub(super) so labels.rs
  can construct and call provider endpoints.
- Adjust wording of a few error messages (e.g. "Assignment request..." -> "Integration request...")
  and genericize some page/result error text.

Export the new labels module from integrations.rs and relax test helper visibility
(pub(crate)) so the labels tests can reuse the existing fixture utilities.

This commit introduces provider-specific parsing and payload logic for labels and
reuses the shared IntegrationApi to perform authenticated requests.

src-tauri/src/main.rs

@@ -38,6 +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,
     list_integration_assignees, get_integration_assignees, set_integration_assignees,
+    list_integration_labels, get_integration_issue_labels, set_integration_issue_labels,
     create_integration_issue, list_azure_issue_projects, list_azure_issue_types,
     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,
 };
@@ -457,6 +458,9 @@ async fn main() {
             move_integration_board_card,
             list_integration_issue_comments,
             add_integration_issue_comment,
+            list_integration_labels,
+            get_integration_issue_labels,
+            set_integration_issue_labels,
             list_integration_assignees,
             get_integration_assignees,
             set_integration_assignees,