Unify date_bucket granularities across aggregation engines

What does this MR do and why?

The date_bucket dimension used in ClickHouse aggregation engines takes a granularity parameter, but until now each engine had to declare its own list of allowed values. This led to drift: some engines allowed daily/weekly/monthly, others only weekly/monthly, or only monthly, and most left the GraphQL argument undocumented.

This MR gives date_bucket a built-in default granularity parameter that accepts daily, weekly, and monthly, with a proper description. All eight engines that use date_bucket now rely on this default instead of repeating their own version. An engine can still override it with its own parameters: if it needs something different (for example the dynamic Xd day granularity, which is intentionally not part of the default).

As a result, two engines gain granularities they didn't support before (agent platform sessions gains daily; code suggestions gains daily and weekly), and every granularity argument is now documented in the GraphQL reference. Previously valid values keep working, so this is backwards compatible.

References

• Related to #609138 (items 4 and 5: default parameter set on date_bucket, and unifying existing engines)
• Related to !251628 (merged) (added the dynamic Xd day granularity that engines can still opt into via a custom parameters: declaration)

Screenshots or screen recordings

Not applicable, no UI changes.

How to set up and validate locally

1. Run the spec file (ClickHouse must be running in GDK):

   bundle exec rspec spec/lib/gitlab/database/aggregation/click_house/date_bucket_dimension_spec.rb

2. In rails console, check the generated parameter for any engine, for example:

   Analytics::AggregationEngines::CodeSuggestions.dimensions.find { |d| d.name == :timestamp }.parameters

   This should return the default granularity hash with daily, weekly, and monthly.
3. In GraphiQL at http://gdk.test:3000/-/graphql-explorer, run an aggregation query with a granularity argument that was previously rejected for code suggestions:

   query {
     group(fullPath: "gitlab-org") {
       analytics {
         duoCodeSuggestions {
           aggregated {
             nodes {
               dimensions {
                 timestamp(granularity: "weekly")
               }
               shownCount
             }
           }
         }
       }
     }
   }

   Confirm weekly is now accepted, and that the docs panel shows the new description for the granularity argument.

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Related to #609138