feat(api): support Grok subscription image generation

[QuangBlue]

Summary

Grok subscription OAuth connections previously could not resolve image models through /v1/images/generations. Register Grok Imagine Image 2.0 and the existing Imagine models for grok-cli/gc and xai-oauth/xao, and add Image 2.0 to xAI API-key connections. Requests reuse the existing OAuth refresh, persistence and account-selection path and dispatch to the xAI image endpoint.

Translate OpenAI pixel sizes into xAI aspect ratio/resolution, preserve explicit native options, map high/hd to medium and standard to low, and validate options before dispatch. Older Imagine models reject the unsupported quality option. Document the callable model IDs and request contract.

Validation

• Change type: provider / API
• Final focused image handler, route, registry and credential retry tests: 73 passed on the company main integration branch
• npm run test:vitest: 54 files / 493 tests passed
• npm run typecheck:core
• npm run lint (full repository) and ESLint on all changed TypeScript files
• node scripts/check/check-api-typecheck.mjs (new diagnostic fixed; existing baseline preserved)
• npm run check:provider-consistency and npm run check:provider-assets
• provider-translate-path-golden.test.ts: 2 passed / 1 failed on macOS; the only snapshot differences are existing Copilot user-agent platform strings (linux versus darwin). This PR does not change the chat provider builders or their golden snapshot.
• npm run check:docs-all
• Pre-commit and commit-message gates
• Full npm run test:unit was started and stopped after failures outside the image change. A separate run on unmodified company main (48bcfc9556) reproduced failures in stream-chunk lifecycle, early keepalive logging, Claude cooldown escalation and relay error handling (25 passed / 6 failed). These files and their implementation are unchanged by this PR.
• CI still has inherited failures: dependency audit in the Lint job (unchanged lockfile), translation drift for docs/architecture/QUALITY_GATES.md, and generated omni-providers skill drift (unchanged generator inputs). The feature guide is a separate document; the API reference translation drift introduced by the initial draft was removed.

Tests Added Or Updated

• tests/unit/image-xai-subscription.test.ts: provider aliases/catalog, native options, size and quality mapping, invalid inputs and legacy model behavior.
• tests/unit/image-generation-route.test.ts: API-key and both OAuth providers, catalog visibility, token refresh persistence and validation before upstream dispatch.

Coverage Notes

The new adapter's branches are exercised by deterministic payload tests; route tests cover credential selection, actual dispatch and persisted token rotation. No credentials or generated media are included in this change.

Reviewer Notes

⚠️ base-red inherited: #15306. The baseline failures above remain outside this feature branch. All focused feature checks pass.

Before implementation, a live subscription OAuth call to https://api.x.ai/v1/images/generations with grok-imagine-image-2.0, aspect_ratio: "3:2", resolution: "1k", quality: "medium" and response_format: "b64_json" returned HTTP 200 and an image. Entitlements and quota remain upstream-controlled.

Provider contract: https://docs.x.ai/developers/model-capabilities/images/generation