fix(project): add transactional plugin deletion lifecycle hook

[muzamilkm]

Summary

DELETE /projects/:projectName deletes core project state directly, but plugins had no deletion lifecycle extension point. A plugin that persists state tied to a DevLake project therefore had no way to validate or clean up that state when deletion went through the core API.

This PR:

• adds an optional plugin.ProjectDeleteHook alongside the existing project-related plugin contracts
• invokes implementing plugins through plugin.TraversalPlugin before core project deletion
• runs plugin cleanup, blueprint cleanup and core project cleanup inside the same database transaction
• adds BlueprintManager.DeleteBlueprintInTransaction so blueprint cleanup can participate in the caller's transaction while preserving the existing DeleteBlueprint(id) entry point
• safely handles projects without an associated blueprint during the unfinished-pipeline check
• adds regression coverage for optional hook dispatch, hook vetoes, rollback and successful single-transaction deletion

Existing plugins do not need to implement ProjectDeleteHook unless they persist state whose lifecycle is tied to a DevLake project.

Does this close any open issues?

Closes #9081

Screenshots

N/A

Other Information

Backwards compatibility

ProjectService remains unchanged and still only requires RenameProject(...).

ProjectDeleteHook is a separate optional plugin interface, so existing plugins and existing ProjectService implementations remain source-compatible.

The existing BlueprintManager.DeleteBlueprint(id) entry point is preserved. DeleteBlueprintInTransaction only provides a transaction-aware path for callers that already own a transaction.

There are no schema, migration, HTTP API, Config UI or Grafana changes in this PR.

Validation

From backend/:

go test ./core/plugin/... -count=1
go test ./helpers/pluginhelper/services/... -count=1
go test ./server/services/... -count=1

go vet ./core/plugin/... ./helpers/pluginhelper/services/... ./server/services/...

gofmt -w core/plugin/plugin_blueprint.go \
  helpers/pluginhelper/services/blueprint_helper.go \
  helpers/pluginhelper/services/blueprint_helper_test.go \
  server/services/project.go \
  server/services/project_test.go

git diff --check devlakeupstream/main...HEAD

All of the above pass locally.

The new hook contract is documented in core/plugin. No user-facing documentation changes are required.