[client] Keep the forced account prompt from being lost to flow reuse

startSSOLogin consumed forceAccountPrompt and applied the prompt to the
freshly built flow, but reuseOAuthFlow could then answer from a cached
flow for the same client — one built without prompt=login, e.g. by
RequestJWTAuth. The user got the same silent authorization URL that
produced the mismatch, with the flag already spent, so no later round
asked either. Rule reuse out when the prompt is forced, while still
cancelling the predecessor's wait.

RequestJWTAuth also wrote the flow fields one by one, leaving the previous
login's hint and accountPrompted behind for WaitSSOLogin to judge a later
token against. Both sites now replace the whole record.

client/server/login_account_test.go

@@ -148,6 +148,26 @@ func TestWaitSSOLogin_JudgesTheFlowThatProducedTheToken(t *testing.T) {
 	require.False(t, s.forceAccountPrompt, "the prompt was armed off another flow's hint")
 }
 
+func TestReuseOAuthFlow_ForcedPromptRefusesTheCachedFlow(t *testing.T) {
+	s := New(internal.CtxInitState(context.Background()), "console", "", false, false, false, false)
+	cancelled := false
+	s.oauthAuthFlow = oauthAuthFlow{
+		flow:       &stubOAuthFlow{},
+		info:       auth.AuthFlowInfo{UserCode: "code"},
+		expiresAt:  time.Now().Add(time.Hour),
+		waitCancel: func() { cancelled = true },
+	}
+
+	state := internal.CtxGetState(s.rootCtx)
+	resp := s.reuseOAuthFlow(context.Background(), &stubOAuthFlow{}, state, true)
+	require.Nil(t, resp, "a forced account prompt reused the flow that skipped it")
+	require.True(t, cancelled, "the predecessor wait was orphaned")
+
+	resp = s.reuseOAuthFlow(context.Background(), &stubOAuthFlow{}, state, false)
+	require.NotNil(t, resp, "an unforced login stopped reusing a live flow")
+	require.Equal(t, "code", resp.UserCode)
+}
+
 func newSSOTestServer(t *testing.T, hint string, accountPrompted bool, tokenEmail string) *Server {
 	t.Helper()
 	s := New(internal.CtxInitState(context.Background()), "console", "", false, false, false, false)