fix(ecstore): replace unwrap() with proper error handling in api_get_object_attributes
Replace unsafe unwrap() calls with proper error handling in
api_get_object_attributes.rs:
- HTTP header access now uses ok_or_else with descriptive messages
- String parsing now uses map_err with descriptive messages
- HeaderValue creation now uses expect with descriptive messages
Refs https://github.com/rustfs/backlog/issues/729