Fix #2795: When using --image with exec codex hangs, and fix image detectio…

[oyiakoumis]

What?

Fixes bug where codex exec --image hangs for 10+ minutes, and resolves inconsistent image handling between TUI and exec.

Why?

Bug 1: codex exec --image ./photo.jpg "describe this" causes indefinite hanging due to race condition in event loop
Bug 2: codex exec "what do you see in ./photo.jpg?" treats image paths as text (shows metadata) while TUI correctly displays images

How?

• Fix race condition: Eliminated competing event consumers that caused --image hanging
• Unify behavior: Added automatic image path detection to exec (matching TUI behavior)
• Refactor: Created shared utilities in codex-common/path_utils to prevent future inconsistencies

Result: Both explicit (--image) and implicit (./image.jpg in text) image references now work consistently across TUI and exec.

Testing

• ✅ codex exec --image ./test.jpg "what do you see?" - no longer hangs
• ✅ codex exec "describe ./test.jpg" - now shows actual image instead of metadata
• ✅ Existing TUI functionality unchanged

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Closes #2795