WebFetch/artifact-read tool also moves to create() instance pattern with new renderToolUseMessage and prompt hook

The WebFetch tool (which fetches a URL and reads its content) has been restructured to use the same create()-based internal pattern already used elsewhere in Claude Code. As part of this it gains a renderToolUseMessage function that, in verbose mode, shows the exact url and prompt being used, and a prompt method for building its own instructions to the model. The way it hands off credentials and abort signals when warming a shared cache entry also changes, now passing an explicit object with toolUseId, abortController, and credentials instead of the whole tool context.