Add MiniMax image generation adapter

[octo-patch]

Reason: Add a native MiniMax image-01 adapter for the documented image_generation endpoint and image_urls response.

• Add the MiniMax image protocol and route requests to /v1/image_generation.
• Serialize image-01 prompts with URL output and parse data.image_urls.
• Reject unsupported edit inputs for this generation-only API.
• Add protocol request and response parsing coverage.

Checks: bun test src/tools/ImageGenTool/providerProtocols.test.ts src/services/imageGeneration/config.test.ts

[github-actions]

PR quality triage

Changed areas: area:cli-core, area:server

CLI core policy: Blocked by policy until a maintainer applies allow-cli-core-change and approves the PR.

Missing-test policy: Blocked by policy until a maintainer applies allow-missing-tests or matching tests are added.

Coverage baseline policy: No coverage-baseline policy block detected.

CLI core files:

• src/tools/ImageGenTool/backend.ts
• src/tools/ImageGenTool/providerProtocols.test.ts
• src/tools/ImageGenTool/providerProtocols.ts

Coverage policy files:

• none

Required check plan:

• See PR Quality / scope-plan; it is the single source of truth for selected jobs.

Test coverage signals:

• BLOCKING unless allow-missing-tests is applied: Server product files changed without a server test file in the PR.
• BLOCKING unless allow-missing-tests is applied: Root runtime product files changed without a matching root runtime test file in the PR.
• Agent/model runtime path changed: use mock/request-shape tests in PR and maintainer live-model smoke before release.

Risk notes:

• Provider/search behavior changed: PR gate uses mock tests; live-provider tests stay maintainer-only.

Hard merge gates come from the deterministic GitHub Actions contract lanes above.