`chat_databricks()` barfs when tool calls are registered

[atheriel]

It seems that Databricks's tool serialization is not OpenAI compatible after all:

> chat <- chat_databricks(model = "databricks-claude-3-7-sonnet")
> chat$register_tool(tool(
  function() format(Sys.Date()),
  .name = "current_date",
  .description = "Returns the current date in ISO 8601 format."
))
> chat$chat("What's the current date in Y-M-D format?")
Error in `req_perform_connection()` at elmer/R/httr2.R:37:5:
! HTTP 400 Bad Request.
• tools.0.custom.strict: Extra inputs are not permitted

Reported by @blairj09.