Add non-destructive provider sync and generic multimodal gateway routing

[AlfredQuan]

Summary

This PR makes provider sync non-destructive by default and adds a generic multimodal gateway compatibility route for Chat Completions upstreams.

Changes

• Default provider sync no longer rewrites historical rollout model_provider values.
• SQLite provider rows are no longer bulk-rewritten during ordinary sync.
• Destructive provider rewrite remains available only as an explicit repair mode.
• Adds routeMode = "multimodalGatewayCompat" for generic multimodal gateway compatibility.
• Converts Responses requests to Chat Completions while replacing image blocks with injected image analysis text.
• Optionally calls a configured vision upstream before the main request.
• Blocks gpt-* models in multimodal gateway mode so OpenAI GPT models stay on official/OpenAI-compatible providers.
• Adds UI fields for route mode, vision model, vision base URL, and GPT blocking.

Validation

• git diff --check
• apps/codex-plus-manager/node_modules/.bin/tsc --noEmit -p apps/codex-plus-manager/tsconfig.json
• cargo test -p codex-plus-core multimodal_gateway --test protocol_proxy
• cargo test -p codex-plus-core settings
• cargo test -p codex-plus-data provider_sync --test provider_sync

[BigPizzaV3]

这个 PR 的方向有用，不过我看到一个需要先修的问题：blockGptOnMultimodalGateway 现在像是“假开关”。

前端新增了“在多模态网关中阻止 gpt-* 模型”的勾选项，也会把 blockGptOnMultimodalGateway 存到 profile 里；但后端实际转换路径里 validate_multimodal_gateway_model() 是无条件阻止 gpt-* / openai/gpt-*：

• RelayProfile.block_gpt_on_multimodal_gateway 有字段；
• UI 也能切换；
• 但 protocol_proxy.rs 没有读取这个字段；
• 所以用户关闭开关后仍然会被阻止。

建议二选一：

1. 让后端按 relay.block_gpt_on_multimodal_gateway 决定是否调用 GPT block；或者
2. 如果产品上必须永远阻止 GPT，就去掉这个 UI 开关，改成固定说明。

另外这个 PR 没有 GitHub checks，修完后建议跑一下：

• cargo test -p codex-plus-core multimodal_gateway --test protocol_proxy
• cargo test -p codex-plus-core settings
• cargo test -p codex-plus-data provider_sync --test provider_sync
• manager 的 TS check