Add BilibiliHotTool for fetching trending Chinese video content

[Tree005]

Summary

I'd like to add a small BilibiliHotTool (~130 LOC) that wraps Bilibili's
public /x/web-interface/popular API, so smolagents Agents can fetch
trending Chinese videos without writing one-off HTTP calls.

Motivation

smolagents' built-in web tools (WebSearchTool, VisitWebpageTool,
WikipediaSearchTool) are all English-centric. As a Chinese content creator
using smolagents for trend research, I keep having to write raw requests.get
calls to the Bilibili API.

The popular endpoint requires no authentication, returns ~20 trending
videos per call (title, UP master, view / danmaku / like counts), and fits
the same role as WikipediaSearchTool does for English Wikipedia.

Implementation plan

• Inherits from Tool, follows the style of WikipediaSearchTool
• Single input: limit (1–50, default 10, clamped to bounds)
• Markdown-formatted output; rate-limited (1 qps default, configurable)
• Network errors returned as descriptive strings (no raise) so the Agent
  can recover
• 8 unit tests + 1 end-to-end example
• No new dependencies (requests already a transitive dep)

Status

I have a working implementation in my fork:
https://github.com/Tree005/smolagents/tree/feat/bilibili-hot-tool

Happy to open a PR once maintainers confirm the direction, or to adjust
naming, output format, or error messages per maintainer preference.

[Tree005]

I've submitted a PR for this: #2671

Happy to adjust the design (naming, output format, error messages, etc.)
based on maintainer feedback. Thanks for considering!