chore: move chatd and related packages to /x/ subpackage (#23445)

- Moves `coderd/chatd/`, `coderd/gitsync/`, `enterprise/coderd/chatd/`
under `x/` parent directories to signal instability
- Adds `Experimental:` glue code comments in `coderd/coderd.go`

> 🤖 This PR was created with the help of Coder Agents, and was
reviewed by my human. 🧑‍💻

coderd/x/chatd/chattool/execute_internal_test.go

@@ -0,0 +1,100 @@
+package chattool
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+	"testing"
+	"unicode/utf8"
+
+	"charm.land/fantasy"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/mock/gomock"
+
+	"github.com/coder/coder/v2/codersdk/workspacesdk"
+	"github.com/coder/coder/v2/codersdk/workspacesdk/agentconnmock"
+	"github.com/coder/coder/v2/testutil"
+)
+
+func TestTruncateOutput(t *testing.T) {
+	t.Parallel()
+
+	t.Run("EmptyOutput", func(t *testing.T) {
+		t.Parallel()
+		result := runForegroundWithOutput(t, "")
+		assert.Empty(t, result.Output)
+	})
+
+	t.Run("ShortOutput", func(t *testing.T) {
+		t.Parallel()
+		result := runForegroundWithOutput(t, "short")
+		assert.Equal(t, "short", result.Output)
+	})
+
+	t.Run("ExactlyAtLimit", func(t *testing.T) {
+		t.Parallel()
+		output := strings.Repeat("a", maxOutputToModel)
+		result := runForegroundWithOutput(t, output)
+		assert.Equal(t, maxOutputToModel, len(result.Output))
+		assert.Equal(t, output, result.Output)
+	})
+
+	t.Run("OverLimit", func(t *testing.T) {
+		t.Parallel()
+		output := strings.Repeat("b", maxOutputToModel+1024)
+		result := runForegroundWithOutput(t, output)
+		assert.Equal(t, maxOutputToModel, len(result.Output))
+	})
+
+	t.Run("MultiByteCutMidCharacter", func(t *testing.T) {
+		t.Parallel()
+		// Build output that places a 3-byte UTF-8 character
+		// (U+2603, snowman ☃) right at the truncation boundary
+		// so the cut falls mid-character.
+		padding := strings.Repeat("x", maxOutputToModel-1)
+		output := padding + "☃" // ☃ is 3 bytes, only 1 byte fits
+		result := runForegroundWithOutput(t, output)
+		assert.LessOrEqual(t, len(result.Output), maxOutputToModel)
+		assert.True(t, utf8.ValidString(result.Output),
+			"truncated output must be valid UTF-8")
+	})
+}
+
+// runForegroundWithOutput runs a foreground command through the
+// Execute tool with a mock that returns the given output, and
+// returns the parsed result.
+func runForegroundWithOutput(t *testing.T, output string) ExecuteResult {
+	t.Helper()
+	ctrl := gomock.NewController(t)
+	mockConn := agentconnmock.NewMockAgentConn(ctrl)
+
+	mockConn.EXPECT().
+		StartProcess(gomock.Any(), gomock.Any()).
+		Return(workspacesdk.StartProcessResponse{ID: "proc-1"}, nil)
+	exitCode := 0
+	mockConn.EXPECT().
+		ProcessOutput(gomock.Any(), "proc-1", gomock.Any()).
+		Return(workspacesdk.ProcessOutputResponse{
+			Running:  false,
+			ExitCode: &exitCode,
+			Output:   output,
+		}, nil)
+
+	tool := Execute(ExecuteOptions{
+		GetWorkspaceConn: func(_ context.Context) (workspacesdk.AgentConn, error) {
+			return mockConn, nil
+		},
+	})
+	ctx := testutil.Context(t, testutil.WaitMedium)
+	resp, err := tool.Run(ctx, fantasy.ToolCall{
+		ID:    "call-1",
+		Name:  "execute",
+		Input: `{"command":"echo test"}`,
+	})
+	require.NoError(t, err)
+
+	var result ExecuteResult
+	require.NoError(t, json.Unmarshal([]byte(resp.Content), &result))
+	return result
+}