feat: support model fallback configuration

[xgtcode]

Summary

• Add VeADK framework support for model fallback configuration via model_fallbacks.
• Support same-provider fallback models and cross-provider fallback endpoints in Studio custom agent creation.
• Generate fallback configuration into agent code, YAML/env handling, and runtime debug metadata.
• Add fallback UI with ordered fallback rows, Ark model dropdown support, ephemeral API key handling, and localized EN/ZH copy.
• Rebuild bundled veadk/webui assets.

Test Plan

• uv run pre-commit run --all-files
• npm test -- tests/modelFallbacks.test.mjs tests/deploymentEnv.test.mjs tests/agentDraftStorage.test.mjs tests/modelSelector.test.mjs
• npm run check:i18n
• npm run build
• uv run pytest tests/cli/test_generated_agent_backend_codegen.py -k "model_fallbacks or custom_model_agent"
• uv run pytest tests/test_agent.py -k "fallback or model_creation"
• uv run pytest tests/runtime/differential/test_runtime_parity.py -k "fallbacks_warn or plain_codex or snapshot"

Notes

• Fallback execution follows the order configured by the user.
• Cross-provider fallback API keys are not persisted in browser draft storage.
• Codex/piagent runtime compatibility currently warns when model_fallbacks cannot be preserved.