Add web_search_2025_08_26 Type Support

[aakankshaduggal]

🚀 Describe the new functionality needed

Currently, Llama Stack supports web_search, web_search_preview, and web_search_preview_2025_03_11 as valid type values for the web_search tool. To improve OpenAI API compatibility, we need to also support web_search_2025_08_26, which is the newer type value used by OpenAI.

💡 Why is this needed? What if we don't build it?

https://llamastack.github.io/docs/providers/openai_responses_limitations#web-search-tool-compatibility

OpenAI API Compatibility: OpenAI's Responses API uses web_search_2025_08_26 as the current web search tool type. Without this support, code written for OpenAI's API won't work with Llama Stack, creating a compatibility gap.

User Experience: Developers migrating from OpenAI or writing cross-platform code expect the same type values to work. Missing web_search_2025_08_26 forces workarounds or prevents adoption.

• Users will encounter validation errors when using OpenAI-compatible code
• Llama Stack will be seen as less compatible with OpenAI's API
• Users may need to modify their code to use older type values like web_search, reducing portability

Other thoughts

No response