feat(web): add Metaso as a web search provider (metaso.cn)

[mrluanma]

Summary

Adds Metaso AI Search as a new SearchProvider option alongside Bing, DuckDuckGo, Tavily, and Bocha.

Testing

• cargo test --all-features
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes

[gemini-code-assist]

Code Review

This pull request adds Metaso as a new web search provider, implementing the necessary API integration, configuration options, and documentation. The feedback focuses on the security risk of including a hardcoded default API key, the need for a more generic API key placeholder in the example configuration to avoid provider-specific confusion, and a suggestion to define the full search endpoint as a constant to improve runtime efficiency.

[gemini-code-assist]

Hardcoding API keys, even public ones with rate limits, poses a security risk. This key can be easily extracted and abused by malicious actors, potentially leading to the exhaustion of the daily quota for all users of this application (a form of denial-of-service). If the key is revoked by Metaso, this functionality will break for everyone relying on the default.

A safer approach would be to require users to provide their own key, even if it's free. This would be consistent with how other providers like Tavily and Bocha are handled.

[gemini-code-assist]

The example API key tvly-YOUR_KEY is specific to Tavily. To avoid confusion for users configuring other providers like Bocha or Metaso, it would be better to use a generic placeholder like YOUR_KEY. This would also make it consistent with the change in docs/CONFIGURATION.md.

# api_key = "YOUR_KEY"  # required for tavily, bocha, and metaso (optional for metaso)

[gemini-code-assist]

To avoid runtime string formatting, it's more efficient to define the full search endpoint URL as a constant. You'll need to update its usage in run_metaso_search from format!("{METASO_ENDPOINT}/search") to just METASO_ENDPOINT.

const METASO_ENDPOINT: &str = "https://metaso.cn/api/v1/search";