fix: support human-readable engine argument values

[ZeroLiu2018]

Summary

• Support the backend's integer/string schema for kv_cache_memory_bytes, so users can enter and retain human-readable values such as 8G.
• Render union-typed values as text inputs, preserve their string form, and validate schema patterns before saving.
• Companion backend change: fix: normalize vLLM KV cache size arguments neutree#605.

Test plan

• GitHub Actions test workflow (lint, typecheck, i18n checks, and unit tests)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/foundation/components/VariablesInput.tsx	44.44%	5 Missing ⚠️

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