Remove github.copilot.chat.responsesApi.toolSearchTool.enabled setting (#315886)

Tool search is now always enabled for gpt-5.4/gpt-5.5, matching the
messages API path. Aligns the responses API on the same
endpoint.supportsToolSearch capability flag.

Also registers ToolSearchTool for gpt-5.4/gpt-5.5 and the
claude-opus-4.7 variants so model-specific tool gating actually
matches the supported endpoints.