[feature] adds web_search and fetch_url tool calls

docs/api/rest.md

@@ -112,6 +112,12 @@ Behavior notes:
 - For `chatId` calls, server stores only *new* non-assistant messages from provided history to avoid duplicates.
 - Server persists final assistant output and call metadata (`LlmCall`) in DB.
 - Server updates chat-level model metadata on each call: `lastUsedProvider`/`lastUsedModel`; first successful/failed call also initializes `initiatedProvider`/`initiatedModel` if unset.
+- For `openai` and `xai`, backend enables tool use during chat completion with an internal system instruction.
+- Available tool calls for chat: `web_search` and `fetch_url`.
+- `web_search` uses Exa and returns ranked results with per-result summaries/snippets.
+- `fetch_url` fetches a URL and returns plaintext page content (HTML converted to text server-side).
+- When a tool call is executed, backend stores a chat `Message` with `role: "tool"` and tool metadata (`metadata.kind = "tool_call"`), then stores the assistant output.
+- `anthropic` currently runs without server-managed tool calls.
 
 ## Searches
 
@@ -171,7 +177,8 @@ Search run notes:
   "createdAt": "...",
   "role": "system|user|assistant|tool",
   "content": "...",
-  "name": null
+  "name": null,
+  "metadata": null
 }
 ```