Go codegen: per-event-type data structs (fixes #1031)

Replace the flat quicktype-generated Data union struct with per-event-type
data structs, matching the C# codegen approach. Adding a new session event
type can no longer change the struct shape of existing event types.

- Replace generateSessionEvents() in scripts/codegen/go.ts with custom
  codegen that produces 74 per-event data structs, a SessionEventData
  interface, and custom UnmarshalJSON/MarshalJSON on SessionEvent
- Update go/session.go to use idiomatic Go type switches for event dispatch
- Update samples, tests, and doc examples for the new API
- Add type/const aliases for PermissionRequest, Attachment, and related
  types to ease migration
- Add .gitattributes eol=lf for generated files to prevent cross-platform
  line ending diffs
- Re-enable the codegen-check CI workflow (remove go/generated_session_events.go
  revert workaround)
- Add ui, uri, mime to Go initialisms for correct field naming

Author: stephentoub

.gitattributes

@@ -1 +1,8 @@
-.github/workflows/*.lock.yml linguist-generated=true merge=ours
+.github/workflows/*.lock.yml linguist-generated=true merge=ours
+
+# Generated files — keep LF line endings so codegen output is deterministic across platforms.
+nodejs/src/generated/* eol=lf linguist-generated=true
+dotnet/src/Generated/* eol=lf linguist-generated=true
+python/copilot/generated/* eol=lf linguist-generated=true
+go/generated_session_events.go eol=lf linguist-generated=true
+go/rpc/generated_rpc.go eol=lf linguist-generated=true

.github/workflows/codegen-check.yml

@@ -47,11 +47,6 @@ jobs:
 
       - name: Check for uncommitted changes
         run: |
-          # TODO: Remove this when https://github.com/github/copilot-sdk/issues/1031 is fixed
-          # Exclude go/generated_session_events.go from the check — it was intentionally
-          # reverted to avoid a breaking DataContent change (see #1031) and will be
-          # regenerated once that issue is resolved.
-          git checkout -- go/generated_session_events.go 2>/dev/null || true
           if [ -n "$(git status --porcelain)" ]; then
             echo "::error::Generated files are out of date. Run 'cd scripts/codegen && npm run generate' and commit the changes."
             git diff --stat

go/README.md

@@ -57,12 +57,10 @@ func main() {
     // Set up event handler
     done := make(chan bool)
     session.On(func(event copilot.SessionEvent) {
-        if event.Type == "assistant.message" {
-            if event.Data.Content != nil && event.Data.Content.String != nil {
-                fmt.Println(*event.Data.Content.String)
-            }
-        }
-        if event.Type == "session.idle" {
+        switch d := event.Data.(type) {
+        case *copilot.AssistantMessageData:
+            fmt.Println(d.Content)
+        case *copilot.SessionIdleData:
             close(done)
         }
     })
@@ -404,30 +402,22 @@ func main() {
     done := make(chan bool)
 
     session.On(func(event copilot.SessionEvent) {
-        if event.Type == "assistant.message_delta" {
+        switch d := event.Data.(type) {
+        case *copilot.AssistantMessageDeltaData:
             // Streaming message chunk - print incrementally
-            if event.Data.DeltaContent != nil {
-                fmt.Print(*event.Data.DeltaContent)
-            }
-        } else if event.Type == "assistant.reasoning_delta" {
+            fmt.Print(d.DeltaContent)
+        case *copilot.AssistantReasoningDeltaData:
             // Streaming reasoning chunk (if model supports reasoning)
-            if event.Data.DeltaContent != nil {
-                fmt.Print(*event.Data.DeltaContent)
-            }
-        } else if event.Type == "assistant.message" {
+            fmt.Print(d.DeltaContent)
+        case *copilot.AssistantMessageData:
             // Final message - complete content
             fmt.Println("\n--- Final message ---")
-            if event.Data.Content != nil && event.Data.Content.String != nil {
-                fmt.Println(*event.Data.Content.String)
-            }
-        } else if event.Type == "assistant.reasoning" {
+            fmt.Println(d.Content)
+        case *copilot.AssistantReasoningData:
             // Final reasoning content (if model supports reasoning)
             fmt.Println("--- Reasoning ---")
-            if event.Data.Content != nil && event.Data.Content.String != nil {
-                fmt.Println(*event.Data.Content.String)
-            }
-        }
-        if event.Type == "session.idle" {
+            fmt.Println(d.Content)
+        case *copilot.SessionIdleData:
             close(done)
         }
     })

go/client.go

@@ -20,8 +20,8 @@
 //	}
 //
 //	session.On(func(event copilot.SessionEvent) {
-//	    if event.Type == "assistant.message" {
-//	        fmt.Println(event.Data.Content)
+//	    if d, ok := event.Data.(*copilot.AssistantMessageData); ok {
+//	        fmt.Println(d.Content)
 //	    }
 //	})
 //