fix: use gtag get for checkout attribution (#8930)

## Summary

Replace checkout attribution GA identity sourcing from
`window.__ga_identity__` with GA4 `gtag('get', ...)` calls keyed by
remote config measurement ID.

## Changes

- **What**:
  - Add typed global `gtag` get definitions and shared GA field types.
- Fetch `client_id`, `session_id`, and `session_number` via `gtag('get',
measurementId, field, callback)` with timeout-based fallback.
- Normalize numeric GA values to strings before emitting checkout
attribution metadata.
- Update checkout attribution tests to mock `gtag` retrieval and verify
requested fields + numeric normalization.
  - Add `ga_measurement_id` to remote config typings.

## Review Focus

Validate the `gtag('get', ...)` retrieval path and failure handling
(`undefined` fallback on timeout/errors) and confirm analytics field
names match GA4 expectations.

## Screenshots (if applicable)

N/A

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8930-fix-use-gtag-get-for-checkout-attribution-30a6d73d365081dcb773da945daceee6)
by [Unito](https://www.unito.io)

src/platform/telemetry/providers/cloud/GtmTelemetryProvider.test.ts

@@ -0,0 +1,69 @@
+import { beforeEach, describe, expect, it } from 'vitest'
+
+import { GtmTelemetryProvider } from './GtmTelemetryProvider'
+
+describe('GtmTelemetryProvider', () => {
+  beforeEach(() => {
+    window.__CONFIG__ = {}
+    window.dataLayer = undefined
+    window.gtag = undefined
+    document.head.innerHTML = ''
+  })
+
+  it('injects the GTM runtime script', () => {
+    window.__CONFIG__ = {
+      gtm_container_id: 'GTM-TEST123'
+    }
+
+    new GtmTelemetryProvider()
+
+    const gtmScript = document.querySelector(
+      'script[src="https://www.googletagmanager.com/gtm.js?id=GTM-TEST123"]'
+    )
+
+    expect(gtmScript).not.toBeNull()
+    expect(window.dataLayer?.[0]).toMatchObject({
+      event: 'gtm.js'
+    })
+  })
+
+  it('bootstraps gtag when a GA measurement id exists', () => {
+    window.__CONFIG__ = {
+      ga_measurement_id: 'G-TEST123'
+    }
+
+    new GtmTelemetryProvider()
+
+    const gtagScript = document.querySelector(
+      'script[src="https://www.googletagmanager.com/gtag/js?id=G-TEST123"]'
+    )
+    const dataLayer = window.dataLayer as unknown[]
+
+    expect(gtagScript).not.toBeNull()
+    expect(typeof window.gtag).toBe('function')
+    expect(dataLayer).toHaveLength(2)
+    expect(Array.from(dataLayer[0] as IArguments)[0]).toBe('js')
+    expect(Array.from(dataLayer[1] as IArguments)).toEqual([
+      'config',
+      'G-TEST123',
+      {
+        send_page_view: false
+      }
+    ])
+  })
+
+  it('does not inject duplicate gtag scripts across repeated init', () => {
+    window.__CONFIG__ = {
+      ga_measurement_id: 'G-TEST123'
+    }
+
+    new GtmTelemetryProvider()
+    new GtmTelemetryProvider()
+
+    const gtagScripts = document.querySelectorAll(
+      'script[src="https://www.googletagmanager.com/gtag/js?id=G-TEST123"]'
+    )
+
+    expect(gtagScripts).toHaveLength(1)
+  })
+})

src/platform/telemetry/providers/cloud/GtmTelemetryProvider.ts

@@ -22,13 +22,21 @@ export class GtmTelemetryProvider implements TelemetryProvider {
     if (typeof window === 'undefined') return
 
     const gtmId = window.__CONFIG__?.gtm_container_id
-    if (!gtmId) {
+    if (gtmId) {
+      this.initializeGtm(gtmId)
+    } else {
       if (import.meta.env.MODE === 'development') {
         console.warn('[GTM] No GTM ID configured, skipping initialization')
       }
-      return
     }
 
+    const measurementId = window.__CONFIG__?.ga_measurement_id
+    if (measurementId) {
+      this.bootstrapGtag(measurementId)
+    }
+  }
+
+  private initializeGtm(gtmId: string): void {
     window.dataLayer = window.dataLayer || []
 
     window.dataLayer.push({
@@ -44,6 +52,38 @@ export class GtmTelemetryProvider implements TelemetryProvider {
     this.initialized = true
   }
 
+  private bootstrapGtag(measurementId: string): void {
+    window.dataLayer = window.dataLayer || []
+
+    if (typeof window.gtag !== 'function') {
+      function gtag() {
+        // gtag queue shape is dataLayer.push(arguments)
+        // eslint-disable-next-line prefer-rest-params
+        ;(window.dataLayer as unknown[] | undefined)?.push(arguments)
+      }
+
+      window.gtag = gtag as Window['gtag']
+    }
+
+    const gtagScriptSrc = `https://www.googletagmanager.com/gtag/js?id=${measurementId}`
+    const existingGtagScript = document.querySelector(
+      `script[src="${gtagScriptSrc}"]`
+    )
+
+    if (!existingGtagScript) {
+      const script = document.createElement('script')
+      script.async = true
+      script.src = gtagScriptSrc
+      document.head.insertBefore(script, document.head.firstChild)
+    }
+
+    const gtag = window.gtag
+    if (typeof gtag !== 'function') return
+
+    gtag('js', new Date())
+    gtag('config', measurementId, { send_page_view: false })
+  }
+
   private pushEvent(event: string, properties?: Record<string, unknown>): void {
     if (!this.initialized) return
     window.dataLayer?.push({ event, ...properties })