refactor(coderd): optimize chatdebug (#27129)

Adds a bund of optimizations to chatdebug:

In `coderd/x/chatd/chatdebug`:
- Adds a benchmark (excluding LLM and database stuff)
- Replaces string concatenation with strings.Builder when accumulating
stream parts (~105,000ns -> ~50,00ns)
- Removes double JSON encode in RecordingTransport (114,000ns ->
64,000ns)

In `coderd/util/strings`:
- Adds a benchmark for Truncate
- Removes unnecessary allocations in Truncate (~110,000ns -> 1,550ns in
truncation case, 1 alloc -> 0 allocs in no truncation case)

> 🤖 Claude helped with this.

coderd/util/strings/strings.go

@@ -64,8 +64,19 @@ func Truncate(s string, n int, opts ...TruncateOption) string {
 	if n < 1 {
 		return ""
 	}
-	runes := []rune(s)
-	if len(runes) <= n {
+
+	// Find the byte offset of the (n+1)th rune, if any; early exit
+	// avoids decoding s past what's needed.
+	runeCount := 0
+	cutoff := -1
+	for i := range s {
+		runeCount++
+		if runeCount > n {
+			cutoff = i
+			break
+		}
+	}
+	if cutoff < 0 {
 		return s
 	}
 
@@ -73,6 +84,7 @@ func Truncate(s string, n int, opts ...TruncateOption) string {
 	if options&TruncateWithEllipsis != 0 {
 		maxLen--
 	}
+	runes := []rune(s[:cutoff])
 	var sb strings.Builder
 	if options&TruncateWithFullWords != 0 {
 		// Convert the rune-safe prefix to a string, then find

coderd/util/strings/strings_test.go

@@ -1,6 +1,7 @@
 package strings_test
 
 import (
+	"bytes"
 	"fmt"
 	"testing"
 
@@ -32,6 +33,7 @@ func TestTruncate(t *testing.T) {
 		{"foo", 1, "f", nil},
 		{"foo", 0, "", nil},
 		{"foo", -1, "", nil},
+		{"", 5, "", nil},
 		{"foo bar", 7, "foo bar", []strings.TruncateOption{strings.TruncateWithEllipsis}},
 		{"foo bar", 6, "foo b…", []strings.TruncateOption{strings.TruncateWithEllipsis}},
 		{"foo bar", 5, "foo …", []strings.TruncateOption{strings.TruncateWithEllipsis}},
@@ -81,6 +83,28 @@ func TestTruncate(t *testing.T) {
 	}
 }
 
+func BenchmarkTruncate(b *testing.B) {
+	b.Run("NoTruncationNeeded", func(b *testing.B) {
+		s := "a short string well under the limit"
+		b.ReportAllocs()
+		for b.Loop() {
+			strings.Truncate(s, 1000)
+		}
+	})
+
+	b.Run("ActualTruncation", func(b *testing.B) {
+		var buf bytes.Buffer
+		for range 2000 {
+			buf.WriteString("日本語テスト word ")
+		}
+		s := buf.String()
+		b.ReportAllocs()
+		for b.Loop() {
+			strings.Truncate(s, 100, strings.TruncateWithEllipsis, strings.TruncateWithFullWords)
+		}
+	})
+}
+
 func TestUISanitize(t *testing.T) {
 	t.Parallel()