Fix redis "multi" use for discussion summarization

We can't use redis "multi" in prod. Remove it.

Test plan:
Discussion summarization should now work in prod

flag=discussion_summary
refs ADV-159

Change-Id: I40eb0e69a42cdbcf12040346501c5e824d2ea3c6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/351959
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/helpers/inst_llm_helper.rb

@@ -51,10 +51,8 @@ module InstLLMHelper
         raise RateLimitExceededError.new(limit:)
       end
 
-      Canvas.redis.multi do |multi|
-        multi.incr(cache_key)
-        multi.expire(cache_key, 24.hours.to_i)
-      end
+      Canvas.redis.incr(cache_key)
+      Canvas.redis.expire(cache_key, 24.hours.to_i)
 
       begin
         yield

spec/helpers/inst_llm_helper_spec.rb

@@ -56,10 +56,6 @@ describe InstLLMHelper do
     let(:user) { double(uuid: "user123") }
     let(:llm_config) { double(rate_limit: { limit: 10, period: "day" }, name: "test") }
 
-    before do
-      allow(Canvas.redis).to receive(:multi).and_yield(double("multi", incr: true, expire: true))
-    end
-
     it "yields if rate limit is not set" do
       llm_config = double(rate_limit: nil)
       expect { |b| InstLLMHelper.with_rate_limit(user:, llm_config:, &b) }.to yield_control
@@ -108,7 +104,6 @@ describe InstLLMHelper do
         Time.now.utc.strftime("%Y%m%d")
       ].cache_key
       allow(Canvas.redis).to receive(:get).with(cache_key).and_return("5")
-      expect(Canvas.redis).to receive(:multi)
       expect { |b| InstLLMHelper.with_rate_limit(user:, llm_config:, &b) }.to yield_control
     end
 
@@ -121,7 +116,6 @@ describe InstLLMHelper do
         Time.now.utc.strftime("%Y%m%d")
       ].cache_key
       allow(Canvas.redis).to receive(:get).with(cache_key).and_return("5")
-      expect(Canvas.redis).to receive(:multi)
       expect(Canvas.redis).to receive(:decr).with(cache_key)
 
       expect do