server, ui: only offer a working directory when a tool reads it (#26762)

The working directory chip showed up as soon as the server exposed any
builtin tool, so a server started with just get_datetime, or a user who
turned every filesystem tool off in the settings, still got a control
that nothing would read.

Tools now declare whether they resolve their paths and run against the
working directory, next to the write permission they already publish in
the /tools listing. The WebUI shows the chip and enables the /cwd
command only when at least one such tool is both served and left
enabled.

tools/ui/tests/client/components/ChatFormPickersHarness.svelte

@@ -21,7 +21,7 @@
 		focusInput: () => {},
 		getShowModelSelector: () => true,
 		hasPrompts: () => true,
-		hasBuiltinTools: () => true,
+		hasCwdTools: () => true,
 		getCwd: () => null,
 		getServerHome: () => null,
 		openModelSelector: () => {