feature/taildrop,ipn/ipn{ext,local}: initialize taildrop for initial profile

Currently, LocalBackend/ExtensionHost doesn't invoke the profile change callback for the initial profile.
Since the initial profile may vary depending on loaded extensions and applied policy settings,
it can't be reliably determined until all extensions are initialized. Additionally, some extensions
may asynchronously trigger a switch to the "best" profile (based on system state and policy settings) during
initialization.

We intended to address these issues as part of the ongoing profileManager/LocalBackend refactoring,
but the changes didn't land in time for the v1.84 release and the Taildrop refactoring.

In this PR, we update the Taildrop extension to retrieve the current profile at initialization time
and handle it as a profile change.

We also defer extension initialization until LocalBackend has started, since the Taildrop extension
already relies on this behavior (e.g., it requires clients to call SetDirectFileRoot before Init).

Fixes #15970
Updates #15812
Updates tailscale/corp#28449

Signed-off-by: Nick Khyl <nickk@tailscale.com>

feature/taildrop/ext.go

@@ -100,6 +100,10 @@ func (e *Extension) Init(h ipnext.Host) error {
 	h.Hooks().SetPeerStatus.Add(e.setPeerStatus)
 	h.Hooks().BackendStateChange.Add(e.onBackendStateChange)
 
+	// TODO(nickkhyl): remove this after the profileManager refactoring.
+	// See tailscale/tailscale#15974.
+	profile, prefs := h.Profiles().CurrentProfileState()
+	e.onChangeProfile(profile, prefs, false)
 	return nil
 }
 

feature/taildrop/integration_test.go

@@ -26,7 +26,6 @@ import (
 // TODO(bradfitz): add test between different users with the peercap to permit that?
 
 func TestTaildropIntegration(t *testing.T) {
-	t.Skip("known failing test; see https://github.com/tailscale/tailscale/issues/15970")
 	testTaildropIntegration(t, false)
 }