Follow-up to the per-provider guards. Three improvements from review:
1. Extract agent.file_safety.raise_if_read_blocked() as a single shared
chokepoint and route the OpenAI, OpenRouter, and (newly) xAI image
providers through it, replacing the 3x-duplicated inline try/except.
Fixes the whole bug class: xai/_xai_image_field read a model-supplied
local path via open() with no guard — the same vulnerability the PR
fixed for OpenAI/OpenRouter, in a sibling provider it missed.
2. Strengthen the regression tests from pass-on-any-ValueError to true
security invariants: spy open()/read_bytes() and assert the blocked
credential is NEVER read; add negative controls (legit local image
still loads; remote/data: URIs pass through unguarded) so a
block-everything regression can't pass.
3. Guard is best-effort by design (defense-in-depth, not a security
boundary) — documented on the shared helper.
- agent/file_safety.py: raise_if_read_blocked()
- plugins/image_gen/{openai,openrouter,xai}: route through helper
- tests: no-read spies + negative controls across all three providers
c1826e2690