Implement Bing Web Search API integration (PH-088-A)

[spaarke-dev]

Context

4 TODO comments in WebSearchTools.cs reference PH-088-A for replacing mock web search results with actual Bing API calls.

Affected Files

• src/server/api/Sprk.Bff.Api/Services/Ai/Chat/Tools/WebSearchTools.cs (lines 13, 53, 76, 111)

What Needs to Happen

1. Provision Bing Search API in Azure
2. Replace mock SearchWebAsync with real API call
3. Remove GenerateMockResults helper method

Source: Code quality audit task 031.