feat: Add first response time distribution report endpoint (#13400)

The index is already added in production.

Adds a new reporting API that returns conversation counts grouped by
channel type and first response time buckets (0-1h, 1-4h, 4-8h, 8-24h,
24h+).

- GET /api/v2/accounts/:id/reports/first_response_time_distribution
- Uses SQL aggregation to handle large datasets efficiently
- Adds composite index on reporting_events for query performance

Tested on production workload.
Request: GET
`/api/v2/accounts/1/reports/first_response_time_distribution?since=<since>&until=<until>`
Response payload:
```
{
    "Channel::WebWidget": {
      "0-1h": 120,
      "1-4h": 85,
      "4-8h": 32,
      "8-24h": 12,
      "24h+": 3
    },
    "Channel::Email": {
      "0-1h": 12,
      "1-4h": 28,
      "4-8h": 45,
      "8-24h": 35,
      "24h+": 10
    },
    "Channel::FacebookPage": {
      "0-1h": 50,
      "1-4h": 30,
      "4-8h": 15,
      "8-24h": 8,
      "24h+": 2
    }
  }
```

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

spec/enterprise/services/enterprise/billing/topup_checkout_service_spec.rb

@@ -46,7 +46,7 @@ describe Enterprise::Billing::TopupCheckoutService do
     it 'raises error for invalid credits' do
       expect do
         service.create_checkout_session(credits: 500)
-      end.to raise_error(Enterprise::Billing::TopupCheckoutService::Error)
+      end.to(raise_error { |error| expect(error.class.name).to eq('Enterprise::Billing::TopupCheckoutService::Error') })
     end
 
     it 'raises error when account is on free plan' do
@@ -54,7 +54,7 @@ describe Enterprise::Billing::TopupCheckoutService do
 
       expect do
         service.create_checkout_session(credits: 1000)
-      end.to raise_error(Enterprise::Billing::TopupCheckoutService::Error)
+      end.to(raise_error { |error| expect(error.class.name).to eq('Enterprise::Billing::TopupCheckoutService::Error') })
     end
   end
 end