QVAC-23970 feat[api]: describe NMT modelConfig union arms

[lauripiisang]

🎯 What problem does this PR solve?

• The NMT (nmtcpp-translation) load-time modelConfig — a discriminated union of the Bergamot and IndicTrans arms plus shared generation params — shipped without descriptions, so the generated contract (consumed by qvac configure and the generated clients) couldn't say what any field means.

📝 How does it solve it?

• Adds .describe() to the shared generation params (mode, beamsize, lengthpenalty, maxlength, repetitionpenalty, norepeatngramsize, temperature, topk, topp), both engine arms (engine, from, to; Bergamot srcVocabSrc/dstVocabSrc/normalize/pivotModel), and the pivot-model sub-fields.
• Semantics sourced from @qvac/translation-nmtcpp README. Descriptions state this schema's transform defaults (temperature 0.3, topk 0, maxlength 512, …), not the README's example values.
• Generation params have full effect on IndicTrans2; Bergamot has limited generation-parameter support (noted in-file). mode is a JS-side selector currently fixed to 'full'.
• Regenerates contract/schema.json and the Python generated models.

Part of QVAC-23933.

🧪 How was it tested?

• bun run contract:export regenerated; contract:check, typecheck, lint, format, and full test:unit (incl. contract-export freshness) pass.
• packages/inference typecheck + lint pass.
• packages/sdk-python generate.py --check passes.

🔌 API Changes

Additive only:

// nmtConfigBaseSchema.options[0] is the Bergamot arm
nmtConfigBaseSchema.options[1].shape.temperature.description
// "Sampling temperature (0–2). Default 0.3."

[github-actions]

Review Status

Current Status: ❌ PENDING
Approvals so far: none

Pending reviews: Needs 1 Management or Team Lead, and 1 more from Management, Team Lead, or Member.

[github-actions]

License compliance — clean

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• ./.github/actions/release-merge-guard
• ./docs/website
• ./packages/ggml-coload-smoke
• ./packages/fabric/test/integration
• ./packages/inference-addon-cpp/mobile
• ./packages/sdk/e2e
• ./packages/llm-llamacpp/benchmarks/performance
• ./packages/llm-llamacpp/benchmarks/server
• ./packages/vla-ggml/sim/server
• ./packages/embed-llamacpp/benchmarks/performance
• ./packages/embed-llamacpp/benchmarks/server
• ./packages/asr-ggml/benchmarks/server