[Agent Backend] E2B connection Missing worpskace-isolation sandbox configuration in UI

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### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

Currently E2B Configuration can only setup globally by environment at system start up. This can no serve the case where each workpace should its individual
agent-backend. Neither it does not cover the case ApiKey is used to auth template access.

### 2. Additional context or comments

Support E2B connection Configuration within WebUI, same as LLM-Provider.
Mainly E2B_API_KEY, E2B_API_URL, E2B_DOMAIN. TemplateId should be opt-in，since apply build-draft always create new snapshot.

### 3. Can you help us with this feature?

- [ ] I am interested in contributing to this feature.

Trace the existing LLM-Provider configuration flow in the WebUI and agent backend, then compare it with the current globally initialized E2B configuration. Confirm how workspace-level E2B_API_KEY, E2B_API_URL, and E2B_DOMAIN settings should work, including optional TemplateId handling for build-draft; done means these settings can be configured per workspace through the UI.

Written by the indexing model from the issue text.