feat(tools): add native MiniMax image generation

[octo-patch]

Reason: MiniMax image generation is missing from the native image tool registry, preventing agents from using its regional API endpoints and response formats.

Summary

• Add a built-in MiniMax image generation tool with the target models and global and China endpoints.
• Route requests through the typed image boundary, map supported sizes to native aspect ratios, and parse base64 or URL receipts.
• Extend local readiness and durable reconciliation coverage for the new tool.

Checks

• uv run --extra dev ruff check --isolated --select E9,F63,F7,F82 app/services/agent_tools.py app/services/agent_runtime/tool_execution.py app/services/builtin_tool_definitions.py tests/test_agent_tools_typed_image_outcomes_v2.py tests/test_agent_tools_deploy_contracts.py
• uv run --extra dev pytest -q tests/test_agent_tools_typed_image_outcomes_v2.py tests/test_agent_tools_deploy_contracts.py tests/test_builtin_tool_contracts.py (146 passed)
• bash scripts/arch-guard.sh
• git diff --check

Checklist

• Tested locally
• No unrelated changes included