fix(api): handle read_image file errors

[wkzCode]

Summary

• resolve relative image paths from the repository root, matching read_text_file
• return structured tool errors for missing files, directories, and image read failures instead of aborting the API agent run
• preserve ToolReturn with binary image content for successful reads
• add coverage for missing, directory, read-failure, and successful relative-path cases

Motivation

API planner models can occasionally mistype a long image path. BinaryContent.from_path() currently lets FileNotFoundError escape, which terminates the entire agent after earlier physical actions may already have executed. Returning the error to the model makes this behavior consistent with read_text_file and allows the model to recover safely.

Validation

• git diff --check
• python -m compileall -q rpent/planner/api_loop.py tests/test_api_read_image.py
• pytest was not run locally because the available Python environments do not have pydantic_ai/pytest installed

[jx-qiu]

Thanks for the fix! Some minor comments: