fix(mcp): expose subjectReference for text_to_image

[dvd233]

Summary

• expose the existing subjectReference option in the stdio and SSE text_to_image schemas
• advertise the same option in the REST tools/list schema

Why

ImageAPI.generateImage already accepts subjectReference, and both README files document it. However, all three MCP transports omitted the option from their tool schemas. Zod therefore strips it from stdio/SSE calls, while REST clients cannot discover it from tools/list, making the documented character-reference capability unavailable through MCP.

This keeps the transport contracts aligned with the existing API implementation without changing request handling or the MiniMax API payload.

Verification

• pnpm run build
• schema smoke test covering stdio, SSE, and REST tool definitions
• ImageAPI forwarding probe for subjectReference → subject_reference
• git diff --check

The repository's existing pnpm run lint reports formatting failures in untouched baseline files; this change avoids reformatting unrelated code.

Related to #17

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