fix(coderd/x/chatd): prevent invalid tool results from poisoning chat history (#24663)

- **computeruse.go**: Decode base64 screenshot data before storing in
`ToolResponse.Data` (was casting base64 string to bytes without
decoding)
- **chatloop.go**: Re-encode `ToolResponse.Data` to base64 via
`base64.StdEncoding.EncodeToString` instead of `string()` cast
- **mcpclient.go**: UTF-8 validate all text from MCP responses in
`convertCallResult()` using `strings.ToValidUTF8`
- **chatprompt.go (persist)**: Defense-in-depth UTF-8 sanitization of
text and media Text fields before database storage
- **chatprompt.go (replay)**: Antivenom layer that validates base64 and
UTF-8 at read time, auto-healing already-poisoned chats without
requiring a migration
- `TestToolResultAntivenom`: 4 subtests covering poisoned text, poisoned
media, valid media round-trip, and media with invalid UTF-8 text
-  Adds `TestConvertCallResult_UTF8Sanitization`: 4 subtests covering invalid
UTF-8 in TextContent, EmbeddedResource, valid passthrough, and
multi-part
- Adds `TestComputerUseTool_Run_ScreenshotDataIsDecodedBinary`: Verifies no
double-encode in the computer-use path
- Updated existing computer-use tests for the new decoded-binary
contract

> 🤖

coderd/x/chatd/chatloop/chatloop.go

@@ -3,6 +3,7 @@ package chatloop
 import (
 	"context"
 	"database/sql"
+	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"maps"
@@ -1213,13 +1214,13 @@ func executeSingleTool(
 		)
 	case resp.Type == "image" || resp.Type == "media":
 		result.Result = fantasy.ToolResultOutputContentMedia{
-			Data:      string(resp.Data),
+			Data:      base64.StdEncoding.EncodeToString(resp.Data),
 			MediaType: resp.MediaType,
-			Text:      resp.Content,
+			Text:      strings.ToValidUTF8(resp.Content, "\uFFFD"),
 		}
 	default:
 		result.Result = fantasy.ToolResultOutputContentText{
-			Text: resp.Content,
+			Text: strings.ToValidUTF8(resp.Content, "\uFFFD"),
 		}
 	}
 	return result