test: assert conf.d stays lazy in non-interactive shells

Spawns one non-interactive child against the sandboxed config and asserts on
what loaded -- abbreviations, key bindings, bang-expansions, the C5 event
handlers and the tailscale completion machinery must all be absent. Exit
codes name which guard regressed. No wall-clock assertions, so it cannot
flake.

tests/functional.fish

@@ -97,6 +97,32 @@ function test_vault_dir_honors_override
     test "$got" = /tmp/vault-override-check
 end
 
+function test_conf_d_is_lazy_in_scripts
+    # A non-interactive shell must not load interactive-only conf.d work.
+    # The child inherits XDG_CONFIG_HOME from the sandboxed session, so it
+    # loads the same config under test. Each exit code names one regression.
+    #
+    # Assertion 5 (tailscale) is vacuously true where tailscale is not
+    # installed: conf.d/tailscale.fish returned early on `type -q tailscale`
+    # before this change, and completions/tailscale.fish does the same, so the
+    # function is absent either way. The test still cannot fail wrongly there
+    # -- it just stops proving anything about that one file. A positive
+    # "completions still work" check would need the binary present and would
+    # make the suite machine-dependent, so it stays out.
+    fish -c '
+        abbr -q n; and exit 1
+        functions -q fish_user_key_bindings; and exit 2
+        functions -q expand_bang_all; and exit 3
+        functions -q __fish_config_logging_changed; and exit 4
+        functions -q __tailscale_perform_completion; and exit 5
+        exit 0'
+end
+
+function test_key_bindings_defined
+    # Positive counterpart to assertion 2 above: the guard must not over-fire.
+    functions -q fish_user_key_bindings
+end
+
 function functional_test_main
     set -l names (functions -a | string match 'test_*' | sort)
     set -l failed 0