fix: enforce H3 reference input limit

[shaoohh]

Summary

• enforce MiniMax-H3's combined limit of 12 reference images, videos, and audios
• keep the existing per-media limits of 9 images, 3 videos, and 3 audios
• add regression coverage proving 12 mixed references are accepted and 13 are rejected

Root cause

The H3 validator checked each media type independently but did not check their combined count. A request containing 9 reference images, 3 reference videos, and 3 reference audios therefore passed local validation with 15 total references, even though mixed-reference requests allow at most 12.

Impact

Oversized mixed-reference requests now fail locally with an actionable usage error before a paid generation request is submitted.

Validation

• bun test test/video/v2.test.ts — 12 passed
• bun test — 450 passed
• bun run typecheck
• bun run lint — 0 errors; 1 pre-existing warning in test/sdk/speech.test.ts
• bun run build

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