Add Codex image generation outside Bedrock

[ex3ndr]

Summary

• add an upstream-compatible image_gen tool backed by the Codex images endpoint
• expose it only when the selected provider supplies image-generation capability, leaving AWS Bedrock unchanged
• persist successful PNG output and return image content to the model; propagate API and empty-output failures safely

Upstream reference

Matches openai/codex rust-v0.144.3 (78ad6e6), especially provider capability gating in model-provider/src/provider.rs / amazon_bedrock/mod.rs and request/output handling in ext/image-generation/src/tool.rs.

Verification

• pnpm --filter @slopus/rig exec vitest run --isolate=false sources/runtime/createCodingAssistantAgent.test.ts sources/providers/codex.test.ts sources/tools/codex/image_gen.test.ts sources/providers/routeProviderThroughGym.test.ts (27 passed)
• pnpm --filter @slopus/rig check
• pnpm --filter @slopus/rig build
• pnpm lint
• touched-file oxfmt --check

Happy2 needs no source change: it delegates provider and tool construction to @slopus/rig.