Fix issue where context is not attach on first use (#7790)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1211654189969294/task/1213386554167128?focus=true

### Description
On first time use the page content is not attached.

Repro steps:
* fresh install and go through onboarding to get to the point where
contextual can be used
* go to eg. bbc.com and tap on duck.ai icon to open the contextual sheet
* sheet opens and shows the first time run
* "attach page content" placeholder is shown alongside the chip
suggestions
* tap on the "summarize" suggestion chip OR tap on placeholder
* Expected: content is attached
* ❌ Actual: content is NOT attached

### Steps to test this PR
The following repro steps should now work

Repro steps:
* fresh install and go through onboarding to get to the point where
contextual can be used
* go to eg. bbc.com and tap on duck.ai icon to open the contextual sheet
* sheet opens and shows the first time run
* "attach page content" placeholder is shown alongside the chip
suggestions
* tap on the "summarize" suggestion chip OR tap on placeholder
* Expected: content is attached
* ❌ Actual: content is NOT attached

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Small, localized change to when a JS subscription event is emitted;
main risk is timing/regression in contextual sheet analytics or
page-context attachment triggering.
> 
> **Overview**
> Fixes first-run Duck.ai contextual mode not attaching page content by
triggering page-context collection when the contextual bottom sheet is
shown.
> 
> Moves the page-context subscription event send out of the omnibar
click path into a new `BrowserTabViewModel.collectPageContext()` called
from `BrowserTabFragment.showDuckChatContextualSheet`, and adds unit
tests to verify the event is emitted only when explicitly collecting
context (and not during onboarding).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
7f779d0. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: David González <malmstein@gmail.com>

Author: aitorvs

app/src/main/java/com/duckduckgo/app/browser/BrowserTabFragment.kt

@@ -3431,6 +3431,8 @@ class BrowserTabFragment :
 
         reactToDuckChatContextualSheetResult()
         ensureBrowserIsCompatibleWithContextualSheetState()
+
+        viewModel.collectPageContext()
     }
 
     private fun createNewContextualFragment(tabId: String) {

app/src/main/java/com/duckduckgo/app/browser/BrowserTabViewModel.kt

@@ -4692,6 +4692,13 @@ class BrowserTabViewModel @Inject constructor(
         duckChat.openDuckChat()
     }
 
+    fun collectPageContext() {
+        viewModelScope.launch {
+            val subscriptionEvent = pageContextJSHelper.onContextualOpened()
+            _subscriptionEventDataChannel.send(subscriptionEvent)
+        }
+    }
+
     fun onDuckChatOmnibarButtonClicked(
         query: String?,
         hasFocus: Boolean,
@@ -4711,10 +4718,6 @@ class BrowserTabViewModel @Inject constructor(
                 viewModelScope.launch {
                     if (duckChat.isContextualOnboardingCompleted()) {
                         command.value = Command.ShowDuckAIContextualMode(tabId)
-                        viewModelScope.launch {
-                            val subscriptionEvent = pageContextJSHelper.onContextualOpened()
-                            _subscriptionEventDataChannel.send(subscriptionEvent)
-                        }
                     } else {
                         command.value = Command.ShowDuckAIContextualOnboarding
                     }

app/src/test/java/com/duckduckgo/app/browser/BrowserTabViewModelTest.kt

@@ -8857,6 +8857,37 @@ class BrowserTabViewModelTest {
         verify(mockDuckChat, never()).openDuckChat()
     }
 
+    @Test
+    fun whenCollectPageContextThenPageContextSubscriptionEventSent() = runTest {
+        val expectedEvent = SubscriptionEventData(
+            featureName = PAGE_CONTEXT_FEATURE_NAME,
+            subscriptionName = "collect",
+            params = JSONObject(),
+        )
+        whenever(mockPageContextJSHelper.onContextualOpened()).thenReturn(expectedEvent)
+
+        testee.collectPageContext()
+
+        testee.subscriptionEventDataFlow.test {
+            val emittedEvent = awaitItem()
+            assertEquals(PAGE_CONTEXT_FEATURE_NAME, emittedEvent.featureName)
+            cancelAndIgnoreRemainingEvents()
+        }
+    }
+
+    @Test
+    fun whenOnDuckChatOmnibarButtonClickedAndOnboardingNotCompletedThenNoPageContextSubscriptionEventSent() = runTest {
+        mockDuckAiContextualModeFlow.emit(true)
+        whenever(mockDuckChat.isContextualOnboardingCompleted()).thenReturn(false)
+
+        testee.subscriptionEventDataFlow.test {
+            testee.onDuckChatOmnibarButtonClicked(query = "example", hasFocus = false, isNtp = false)
+
+            expectNoEvents()
+            cancelAndIgnoreRemainingEvents()
+        }
+    }
+
     @Test
     fun whenSubscriptionChangesWhileOnDuckAiThenAuthUpdateEventSent() = runTest {
         val duckAiUrl = "https://duck.ai/chat"