fix(image-gen): resolve relative URLs returned by Xinference and compatible backends

[okxint]

When using an OpenAI-compatible image backend like Xinference, the API can return a relative URL like /files/image.png instead of an absolute one. The browser can't resolve this, so generated images fail to display.

Root cause: generate_image.py passed data[0].url directly to the frontend without checking whether it was absolute. Xinference's file-serving endpoint returns a path-only URL.

Fix: After resolving the model config via to_litellm(), we now capture api_base. When the returned URL starts with /, we extract the origin (scheme + host + port) from api_base and prepend it to produce a fully-qualified URL. Absolute URLs (OpenAI, Azure, Recraft, etc.) are passed through unchanged.

No behaviour change for providers that already return absolute URLs.

Closes #1496

High-level PR Summary

This PR fixes an issue where OpenAI-compatible image generation backends like Xinference return relative URLs (e.g., /files/image.png) instead of absolute URLs, causing images to fail to display in the browser. The fix captures the api_base from the resolved model configuration and prepends the provider's origin (scheme + host + port) to relative URLs, while leaving absolute URLs unchanged. This ensures all image URLs are fully-qualified and browser-resolvable.

⏱️ Estimated Review Time: 5-15 minutes

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Order	File Path
1	surfsense_backend/app/agents/chat/multi_agent_chat/subagents/builtins/deliverables/tools/generate_image.py

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