[client] Force interactive login when extending the auth session

A session extend must be answered from the account the peer is registered
under. With a silent PKCE flow (DisablePromptLogin or max_age=0) the IdP
answers from whatever session it already holds, which need not be the
peer's account when several are signed in; the token then fails the
user match in ExtendAuthSession with no way to pick another account.

Mark the PKCE flow request as a session extend so the management server
can force prompt=login for it, overriding the configured silent flow.

client/android/login.go

@@ -199,7 +199,15 @@ type loginHintSetter interface {
 }
 
 func (a *Auth) foregroundGetTokenInfo(authClient *auth.Auth, urlOpener URLOpener, isAndroidTV bool) (*auth.TokenInfo, error) {
-	oAuthFlow, err := authClient.GetOAuthFlow(a.ctx, isAndroidTV)
+	return a.foregroundGetTokenInfoFlow(authClient, urlOpener, isAndroidTV, false)
+}
+
+// foregroundGetTokenInfoFlow runs the interactive flow. sessionExtend tells the
+// server the token will renew this peer's session rather than log a peer in, so
+// it can rule out a silent authorization the IdP could answer from an unrelated
+// account. See PKCEAuthorizationFlowRequest.
+func (a *Auth) foregroundGetTokenInfoFlow(authClient *auth.Auth, urlOpener URLOpener, isAndroidTV bool, sessionExtend bool) (*auth.TokenInfo, error) {
+	oAuthFlow, err := authClient.GetOAuthFlow(a.ctx, isAndroidTV, sessionExtend)
 	if err != nil {
 		return nil, fmt.Errorf("failed to get OAuth flow: %v", err)
 	}

client/android/session.go

@@ -293,11 +293,13 @@ func (c *Client) extendAuthSession(ctx context.Context, urlOpener URLOpener, isA
 	}
 	defer authClient.Close()
 
-	// Passing the config path makes the flow pick up the login_hint: an extend
-	// renews the session of the account already signed in, so it must not stop to
-	// offer a choice.
+	// Passing the config path makes the flow pick up the login_hint. That alone
+	// cannot keep the IdP on this profile's account though — a hint is only a
+	// suggestion, and a silent authorization is answered from whatever session the
+	// IdP already has, which need not be this peer's when several accounts are
+	// signed in. Marking the flow as an extend lets the server rule that out.
 	a := NewAuthWithConfig(ctx, cfg, cfgPath)
-	tokenInfo, err := a.foregroundGetTokenInfo(authClient, urlOpener, isAndroidTV)
+	tokenInfo, err := a.foregroundGetTokenInfoFlow(authClient, urlOpener, isAndroidTV, true)
 	if err != nil {
 		return fmt.Errorf("interactive sso login failed: %v", err)
 	}