fix(httpapi): return project not found errors

[nexxeln]

summary

• convert missing project updates to typed Project.NotFoundError at the service layer
• map project update missing IDs to public ProjectNotFoundError bodies
• update OpenAPI, HTTP, project service, and exerciser coverage

testing

• bun test test/project/project.test.ts test/server/httpapi-instance.test.ts test/server/httpapi-public-openapi.test.ts
• bun typecheck
• bun test ./test/server/httpapi-exercise/index.ts
• bun turbo typecheck