Clearer handling and messaging when tool search is unavailable

What

• A new "tool search absent" mode was added for when tool search isn't available for a given model or session.
  • The reasons tool search can be unavailable (model_unsupported, vertex_model_unsupported, foundry_deployment_unsupported, no_tools_in_request, not_registered, mcp_search_unavailable) were consolidated into a single function that returns a reason code, used both by the existing gating logic and a new helper that specifically checks for the mcp_search_unavailable case.
  • The messaging shown for deferred/blocked tools now changes wording when tool search is absent: it says "tool" instead of "deferred tool", says "Do not call them" instead of "Do not search for them", and describes MCP servers still connecting as tools that "will be announced here" rather than tools to search for.
  • The deferred-tools-pool telemetry event (tengu_deferred_tools_pool_change) gained new fields resurfacedCount and toolSearchAbsent, and in this mode resurfaced tools are counted separately.

Why

When tool search can't be used, Claude now gets consistent messaging and reasoning, driven by one shared reason-code function, that matches its actual situation instead of instructions referencing a tool-search feature it can't use.