agent: Refine the web search tool call UI (#29190)

This PR refines a bit the web search tool UI by introducing a component
(`ToolCallCardHeader`) that aims to standardize the heading element of
tool calls in the thread.

In terms of next steps, I plan to evolve this component further soon
(e.g., building a full-blown "tool call card" component), and even move
it to a place where I can re-use it in the active_thread as well without
making the `assistant_tools` a dependency of it.

Release Notes:

- N/A

crates/assistant_tools/src/ui.rs

@@ -0,0 +1,3 @@
+mod tool_call_card_header;
+
+pub use tool_call_card_header::*;