feat(server): impl storage runtime (#15181)

#### PR Dependency Tree


* **PR #15181** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added an additional storage backend option: asset-pack based storage
(provider for avatar, blob, and copilot).
* Introduced a dedicated storage runtime with provider capability
reporting and expanded object operations (put/head/get/list/delete),
including presigned and multipart flows where supported.
* Cloudflare R2 `jurisdiction` now uses an explicit default when
omitted.
* **Bug Fixes**
  * Broadened avatar access to allow both fs and asset-pack providers.
* Improved workspace blob upload completion validation and handling when
stored objects are missing or mismatched.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/native/src/lib.rs

@@ -2,7 +2,6 @@
 
 mod utils;
 
-pub mod backend_runtime;
 pub mod doc;
 pub mod doc_loader;
 pub mod entitlement;
@@ -13,6 +12,7 @@ pub mod image;
 pub mod license;
 pub mod llm;
 pub mod permission;
+pub mod runtime;
 pub mod safe_fetch;
 pub mod tiktoken;