Remove auto-tagging timeline entry (ENG-2436) (#4512)

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<!-- CURSOR_AGENT_PR_BODY_BEGIN -->
## Summary

This PR removes the auto-tagging timeline entry from the change analysis
workflow, as all customers have now migrated to CLI version 1.16.4 or
later.

## Changes

### Core Changes
- **Removed `ChangeTimelineEntryV2IDAutoTagging` from
`PopulateChangeTimelineV2`** in
`services/api-server/service/changeanalysis/changetimeline.go`
  - Timeline entries no longer include auto-tagging step

- **Removed auto-tagging from `GetChangeTimelineV2`** function
  - Removed call to `GetAutoTaggingTimelineEntry`
  - Removed the function `GetAutoTaggingTimelineEntry` entirely

- **Removed `TrackChangeTimelineV2Entry` call for auto-tagging** in
`services/api-server/service/changeanalysis/change_analysis.go`
  - Auto-tagging logic now runs without timeline tracking
  - Added comment noting the timeline entry was removed per ENG-2436

- **Removed auto-tagging label from `resetChangeAnalysisTables`** in
`services/api-server/service/changeanalysis/shared.go`
  - Timeline reset no longer includes auto-tagging entry

- **Removed `ChangeTimelineEntryV2IDAutoTagging` constant** from
`go/sdp-go/changetimeline.go`
  - Removed from `allChangeTimelineEntryV2IDs` slice
- Kept deprecated fields in change_analysis.go for backward
compatibility with old changes

### Test Updates
- Updated `TestChangeTimelineV2` to remove auto-tagging test assertions
- Updated `TestCreateInstantChangeTimelineEntry` to use change
validation instead of auto-tagging
- Updated `TestGetChangeTimelineEntryFromDatabase` expected count from 8
to 7 entries
- Updated `TestPopulateChangeTimelineV2FeatureFlags` expected count from
9 to 8 entries
- Updated `TestGetChangeTimelineV2` expected count from 8 to 7 entries
- Removed auto-tagging test case from `go/sdp-go/changetimeline_test.go`

## Testing

- ✅ All modified Go packages compile successfully
- ✅ Code builds without errors
- Tests require database connection (not available in CI-less
environment)

## Migration Notes

This is a cleanup task following the successful migration of all
customers to CLI version 1.16.4+. The auto-tagging functionality itself
is preserved; only the timeline entry tracking has been removed.

## Checklist

- [x] Removed auto-tagging from PopulateChangeTimelineV2
- [x] Removed auto-tagging from GetChangeTimelineV2
- [x] Removed GetAutoTaggingTimelineEntry function
- [x] Removed TrackChangeTimelineV2Entry call in change_analysis.go
- [x] Removed auto-tagging from resetChangeAnalysisTables
- [x] Removed timeline entry ID constant from SDP
- [x] Updated all affected tests
- [x] Code compiles successfully
<!-- CURSOR_AGENT_PR_BODY_END -->

Linear Issue:
[ENG-2436](https://linear.app/overmind/issue/ENG-2436/remove-auto-tagging-time-line-entry-now-we-are-fully-adopted)

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GitOrigin-RevId: ff9c45adb59683fd3523af1c93b663ff3e42b00a

Author: DavidS-ovm

go/sdp-go/changetimeline.go

@@ -67,11 +67,6 @@ var (
 		Label: "calculated_labels",
 		Name:  "Apply auto labels",
 	}
-	// Tracks the application of auto tags for a change
-	ChangeTimelineEntryV2IDAutoTagging = ChangeTimelineEntryV2ID{
-		Label: "auto_tagging",
-		Name:  "Auto Tagging",
-	}
 	// Tracks the validation of a change. This happens after the change is
 	// complete and at time of writing is not generally available
 	ChangeTimelineEntryV2IDChangeValidation = ChangeTimelineEntryV2ID{
@@ -124,7 +119,6 @@ var allChangeTimelineEntryV2IDs = []ChangeTimelineEntryV2ID{
 	ChangeTimelineEntryV2IDAnalyzedSignals,
 	ChangeTimelineEntryV2IDCalculatedRisks,
 	ChangeTimelineEntryV2IDCalculatedLabels,
-	ChangeTimelineEntryV2IDAutoTagging,
 	ChangeTimelineEntryV2IDChangeValidation,
 	ChangeTimelineEntryV2IDRecordObservations,
 	ChangeTimelineEntryV2IDFormHypotheses,

go/sdp-go/changetimeline_test.go

@@ -52,11 +52,6 @@ func TestChangeTimelineEntryNameConversion(t *testing.T) {
 			entryID:              ChangeTimelineEntryV2IDCalculatedRisks,
 			hasInProgressVariant: false,
 		},
-		{
-			name:                 "Auto Tagging (no in-progress variant)",
-			entryID:              ChangeTimelineEntryV2IDAutoTagging,
-			hasInProgressVariant: false,
-		},
 		{
 			name:                 "Change Validation (no in-progress variant)",
 			entryID:              ChangeTimelineEntryV2IDChangeValidation,