fix(chatd): sanitize \u0000 from JSON before JSONB insertion (#22637)

## Problem

Users hit this error when agent tool results contain Unicode null
characters:

```
persist step: insert tool result: pq: unsupported Unicode escape sequence
```

PostgreSQL's `jsonb` type rejects `\u0000` (Unicode null, U+0000) with
that error, even though it's valid JSON per RFC 8259. Tool results from
agents can contain this sequence — e.g. binary data, C-style strings, or
certain API responses.

## Root cause

`MarshalToolResult` and `MarshalContent` in `chatprompt.go` serialize
content blocks to JSON and pass them directly to `InsertChatMessage`
which casts to `::jsonb`. Go's `json.Marshal` / `json.Valid` accept
`\u0000`, but Postgres does not.

## Fix

Added `sanitizeJSONForPG()` which strips `\u0000` escape sequences from
serialized JSON before insertion. Uses `bytes.Contains` as a fast-path
check to avoid allocation when no null bytes are present (the common
case).

Applied to both `MarshalContent` (assistant messages) and
`MarshalToolResult` (tool result messages).

coderd/chatd/chatprompt/chatprompt.go

@@ -1,6 +1,7 @@
 package chatprompt
 
 import (
+	"bytes"
 	"encoding/json"
 	"regexp"
 	"strings"
@@ -422,6 +423,7 @@ func MarshalContent(blocks []fantasy.Content) (pqtype.NullRawMessage, error) {
 	if err != nil {
 		return pqtype.NullRawMessage{}, xerrors.Errorf("encode content blocks: %w", err)
 	}
+	data = sanitizeJSONForPG(data)
 	return pqtype.NullRawMessage{RawMessage: data, Valid: true}, nil
 }
 
@@ -439,6 +441,7 @@ func MarshalToolResult(toolCallID, toolName string, result json.RawMessage, isEr
 	if err != nil {
 		return pqtype.NullRawMessage{}, xerrors.Errorf("encode tool result: %w", err)
 	}
+	data = sanitizeJSONForPG(data)
 	return pqtype.NullRawMessage{RawMessage: data, Valid: true}, nil
 }
 
@@ -835,6 +838,22 @@ func sanitizeToolCallID(id string) string {
 	return toolCallIDSanitizer.ReplaceAllString(id, "_")
 }
 
+// jsonNullEscape is the JSON escape sequence for Unicode null (U+0000).
+var jsonNullEscape = []byte(`\u0000`)
+
+// sanitizeJSONForPG strips \u0000 escape sequences from JSON data.
+// PostgreSQL's jsonb type rejects the Unicode null character (U+0000)
+// with "unsupported Unicode escape sequence", even though \u0000 is
+// valid JSON per RFC 8259. Tool results from agents may contain this
+// sequence (e.g. binary data or C-style strings), so we strip it
+// before insertion.
+func sanitizeJSONForPG(data []byte) []byte {
+	if bytes.Contains(data, jsonNullEscape) {
+		data = bytes.ReplaceAll(data, jsonNullEscape, nil)
+	}
+	return data
+}
+
 func marshalContentBlock(block fantasy.Content) (json.RawMessage, error) {
 	encoded, err := json.Marshal(block)
 	if err != nil {

coderd/chatd/chatprompt/chatprompt_test.go

@@ -1,6 +1,7 @@
 package chatprompt_test
 
 import (
+	"bytes"
 	"encoding/json"
 	"testing"
 
@@ -89,3 +90,27 @@ func TestConvertMessages_NormalizesAssistantToolCallInput(t *testing.T) {
 		})
 	}
 }
+
+func TestMarshalToolResult_SanitizesNullBytes(t *testing.T) {
+	t.Parallel()
+
+	result := json.RawMessage(`{"output":"hello\u0000world"}`)
+	got, err := chatprompt.MarshalToolResult("call_1", "my_tool", result, false)
+	require.NoError(t, err)
+	require.True(t, got.Valid)
+	require.False(t, bytes.Contains(got.RawMessage, []byte(`\u0000`)),
+		"output should not contain \\u0000 escape sequences")
+}
+
+func TestMarshalContent_SanitizesNullBytes(t *testing.T) {
+	t.Parallel()
+
+	blocks := []fantasy.Content{
+		fantasy.TextContent{Text: "before\u0000after"},
+	}
+	got, err := chatprompt.MarshalContent(blocks)
+	require.NoError(t, err)
+	require.True(t, got.Valid)
+	require.False(t, bytes.Contains(got.RawMessage, []byte(`\u0000`)),
+		"output should not contain \\u0000 escape sequences")
+}