* test: add unit tests for ViewImageMiddleware
- Add 33 test cases covering all 7 internal methods plus sync/async
before_model hooks
- Cover normal path, edge cases (missing keys, empty base64, stale
ToolMessages before assistant turn), and deduplication logic
- Related to Q2 Roadmap #1669
* test: add unit tests for ViewImageMiddleware
Add 35 test cases covering all internal methods, before_model hooks,
and edge cases (missing attrs, list-content dedup, stale ToolMessages).
Related to #1669
