feat: delete user (#637)

## Summary

- Added endpoint to delete a user

## Validation

- go test ./... -tags postgres_integration

## Release notes / changeset

- Changeset: .changeset/delete-user-endpoint.md

## Notes

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: gayathri <66356931+grvijayan@users.noreply.github.com>

Author: 3 people

.changeset/delete-user-endpoint.md

@@ -0,0 +1,6 @@
+---
+"@zitadel/server": minor
+"@zitadel/api": minor
+---
+
+Add `DELETE /users/{user_id}` to delete a user from a project. Requires an OAuth2 bearer with the `user.write` scope and returns `204 No Content`.