Fix generated images in Codex, Account Pool, and thread history

[ymichael]

Human comments

What was wrong

A real Codex image-generation request could fail at several points: Account Pool returned 404 for native image requests, BB rejected Codex's current image event format, and saved generated-image rows had neither an expansion control nor a rendered body. Older successful or failed attempts could also disappear on reload because the history filter required a marker added only when large results were migrated. This completes the image support introduced in #2920 and the pooled Codex transport introduced in #3056.

What changed

• Render generated images through the existing image preview and full-size viewer, with readable fallback bodies for failures or unavailable files.
• Preserve older image completions with small, empty, missing, or null results, while retaining the existing large-result migration behavior. Accept the nullable background flag in retained native attempts as well.
• Accept native in_progress status and nullable transparentBackground while preserving the existing normalized event contract.
• Forward image generation and editing through Account Pool's existing authenticated Codex handler, preserving account selection and credential replacement. Update configuration, CLI guide, and skill documentation.

No server/daemon wire fields change. Native web-search routing is outside this change.

How you verified

• Regression coverage in existing Codex and Account Pool tests fails before the fixes and passes afterward, including image-route authentication, upstream credentials, and unchanged request/response bodies.
• pnpm exec turbo run test typecheck --filter=bb-plugin-account-pool --filter=bb-plugin-provider-codex: 531 tests passed and both typechecks passed.
• Added history regression coverage for short, threshold-sized, Unicode, empty, missing, and null image results across repeated SQLite migration sweeps. All 31 focused server timeline tests and 475 database tests and 201 domain tests passed through Turbo; domain, database, and server typechecks passed.
• After rebasing, pnpm exec turbo run typecheck --filter=@bb/app --filter=@bb/client-core --filter=bb-plugin-provider-codex --filter=bb-plugin-account-pool passed.
• Generated a real 1254×1254 PNG using native Codex through the patched dev Account Pool. Verified the saved-path preview and full-size viewer on desktop 1440×900 and compact 390×844 with a coarse pointer. The isolated QA pool threshold was set to 1 to allow the account's remaining quota; production settings were unchanged.
• Reloaded the earlier real failed Codex request and verified its previously hidden empty-result image attempt now appears and expands to a readable failure body.
• Live verification thread: expand Worked for 57s → Explored 1 file, generated 1 image → Generated image, then click the image. The successful turn follows an initial quota-threshold failure.

AGENT GENERATED