fix(ai template): avoid nullable filenameFilter tool schema for Ollama

[SergioChan]

Summary

• change SearchAsync's filenameFilter parameter in the AI Chat Web template from nullable string to non-null string defaulting to empty
• this keeps the generated function schema compatible with OllamaSharp's current mapper expectation for type: "string"
• preserves existing behavior because empty string already means "search all files"

Testing

• Static validation: confirmed the template now emits a non-nullable string filenameFilter = "" signature at Chat.razor (single-line scoped change)
• Could not run dotnet build in this environment because the dotnet CLI is not installed (dotnet: command not found)

Related

Fixes #7350

[Copilot]

Pull request overview

This PR updates the AI Chat Web C# project template to avoid generating a nullable filenameFilter tool parameter schema, which currently breaks OllamaSharp’s tool mapper during function/tool serialization.

Changes:

• Change the SearchAsync tool parameter filenameFilter from string? = null to non-nullable string = "" to prevent a nullable JSON schema type (array) from being emitted.
• Preserve existing search behavior by continuing to treat empty string as “no filter / search all files”.