fix: resolve image paths to absolute paths

[crazywolf132]

Description

This PR fixes issue #1010 where the --image flag was not working correctly. The problem was that the code was trying to read image files using the path as provided without resolving it to an absolute path, causing issues when the path was relative to the current working directory.

Changes

• Modified the createInputItem function in codex-cli/src/utils/input-utils.ts to resolve relative image paths to absolute paths before attempting to read the file
• Added a check to determine if the path is already absolute, and if not, resolve it relative to the current working directory
• Updated the tests to verify that the path resolution works correctly

Testing

• Added new test cases to verify both relative and absolute path handling
• All linting and type checking tests pass

Related Issues

Fixes #1010