fix: parse Codex extension image generation results

[StreetJammer]

Summary:

• Add support for Codex CLI 0.149.1 image generation rollout records emitted as completed Extension/image_gen.generation items.
• Keep support for legacy image_generation_call and image_generation_end result records.
• Add regression coverage for the new rollout schema.

Tests:

• python -m pytest tests/gen/test_gen.py -q
• python -m pytest tests/gen -q

Notes:
This fixes false “image_gen tool never ran” diagnostics when Codex did generate a valid inline PNG under the newer item_completed Extension schema.

[aldegad]

Thanks for adding support for the Codex 0.149 image_gen.generation Extension record.

I reviewed the full change for security and supply-chain risks. No malicious behavior, dependency changes, workflow changes, new network or subprocess behavior, path trust, binaries, symlinks, or secrets were found.

I added one maintainer follow-up commit so matching failed records and completed records without a result fail explicitly instead of being misreported as "image_gen never ran". The original successful-result behavior is unchanged.

Verification on the updated PR head:

• focused provider tests: 30 passed
• generation tests: 133 passed
• full suite: 969 passed
• git diff check, git fsck, and gitleaks passed

Merging with your original commit and authorship preserved. Thank you, Arun.