feat(rollup): report builder abstraction [2/3] (#13798)

## PR2: Report builder refactor — DataSource abstraction

The existing report builders (timeseries + summary) had their SQL
queries inlined — each builder constructed its own scopes, groupings,
and aggregations directly. This made it hard to swap the underlying data
source without duplicating builder logic.

This PR extracts all raw-event querying into a `Reports::RawDataSource`
behind a `Reports::DataSource` factory. Builders now call
`data_source.timeseries`, `.aggregate`, or `.summary` instead of
constructing queries themselves. Behavior is identical —
`DataSource.for(...)` returns `RawDataSource` in all cases today.

The timeseries path had two separate builders (`CountReportBuilder`,
`AverageReportBuilder`) that were selected via a metric-name case
statement in `Conversations::BaseReportBuilder`. These are replaced by a
single `ReportBuilder` that delegates to the data source. The metric
type (count vs average) is now decided inside the data source, not the
builder.

Summary builders similarly moved their inline SQL into
`RawDataSource#summary`, which returns a unified hash keyed by dimension
ID.
 the rollup read path.

## Flow

### Before

```
ReportsController ──▶ case metric ──▶ AverageReportBuilder ──▶ inline SQL ──▶ DB
                                  └──▶ CountReportBuilder   ──▶ inline SQL ──▶ DB

SummaryController ──▶ AgentSummaryBuilder ──▶ inline SQL ──▶ DB
                  └──▶ InboxSummaryBuilder ──▶ inline SQL ──▶ DB
                  └──▶ TeamSummaryBuilder  ──▶ inline SQL ──▶ DB
```

### After

```
ReportsController ──▶ ReportBuilder  ──┐
                                       ├──▶ DataSource.for ──▶ RawDataSource ──▶ DB
SummaryController ──▶ SummaryBuilder ──┘
```


### Expected (after rollup read path)

```
ReportsController ──▶ ReportBuilder  ──┐
                                       ├──▶ DataSource.for ──▶ RawDataSource    ──▶ reporting_events
SummaryController ──▶ SummaryBuilder ──┘                   └──▶ RollupDataSource ──▶ reporting_events_rollups
```

### What changed

- `Reports::DataSource` factory + `Reports::RawDataSource`
- `TimezoneHelper#timezone_name_from_params` — prefers IANA name, falls
back to offset
- Unified `Timeseries::ReportBuilder` replaces `CountReportBuilder` +
`AverageReportBuilder`
- Summary builders delegate to `DataSource` instead of querying directly

### How to test

This is a pure refactor — all existing report pages (Overview, Agent,
Inbox, Label, Team) should produce identical numbers. No feature flag or
new config needed.

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Tanmay Deep Sharma <tanmaydeepsharma21@gmail.com>
Co-authored-by: Tanmay Deep Sharma <32020192+tds-1@users.noreply.github.com>

spec/controllers/api/v2/accounts/summary_reports_controller_spec.rb

@@ -45,7 +45,10 @@ RSpec.describe 'Summary Reports API', type: :request do
             headers: admin.create_new_auth_token,
             as: :json
 
-        expect(V2::Reports::AgentSummaryBuilder).to have_received(:new).with(account: account, params: params)
+        expect(V2::Reports::AgentSummaryBuilder).to have_received(:new).with(
+          account: account,
+          params: params.merge(type: :agent)
+        )
         expect(agent_summary_builder).to have_received(:build)
 
         expect(response).to have_http_status(:success)
@@ -96,7 +99,10 @@ RSpec.describe 'Summary Reports API', type: :request do
             headers: admin.create_new_auth_token,
             as: :json
 
-        expect(V2::Reports::InboxSummaryBuilder).to have_received(:new).with(account: account, params: params)
+        expect(V2::Reports::InboxSummaryBuilder).to have_received(:new).with(
+          account: account,
+          params: params.merge(type: :inbox)
+        )
         expect(inbox_summary_builder).to have_received(:build)
 
         expect(response).to have_http_status(:success)
@@ -147,7 +153,10 @@ RSpec.describe 'Summary Reports API', type: :request do
             headers: admin.create_new_auth_token,
             as: :json
 
-        expect(V2::Reports::TeamSummaryBuilder).to have_received(:new).with(account: account, params: params)
+        expect(V2::Reports::TeamSummaryBuilder).to have_received(:new).with(
+          account: account,
+          params: params.merge(type: :team)
+        )
         expect(team_summary_builder).to have_received(:build)
 
         expect(response).to have_http_status(:success)