partialParse in _vendor/partial-json-parser crashes on invalid JSON escape sequences from model output

## Bug

When a model emits tool call JSON containing characters like unescaped backslashes (e.g. PHP namespaces `App\Http\Middleware\HandleAppearance`), the `partialParse` function in `_vendor/partial-json-parser/parser.{js,mjs}` throws:

```
Bad escaped character in JSON at position 80 (line 1 column 81)
```

This crashes the `MessageStream` during `input_json_delta` accumulation (line 501 in `MessageStream.mjs`), killing the entire streaming response.

## Root Cause

The `generate()` function reconstructs JSON from tokens but does not sanitize string values. The tokenizer correctly captures escape sequences like `\H` from `App\Http`, but `generate()` outputs them verbatim back into a JSON string:

```js
// generate() outputs: "use App\Http\Middleware" — \H and \M are invalid JSON escapes
output += ' + token.value + ';
```

`JSON.parse()` then rejects `\H`, `\M`, `\A` etc. since only `\" \\ \/ \b \f \n \r \t \uXXXX` are valid JSON escape sequences.

## Fix

In the `generate()` function, sanitize string token values by doubling backslashes that precede invalid escape characters:

```js
case 'string':
// Repair invalid JSON escapes (e.g. \H from PHP namespaces)
let sanitized = token.value.replace(/\\([^"\\\/bfnrtu])/g, \\$1);
output += " + sanitized + ";
break;
```

This turns `\H` → `\\H`, `\M` → `\\M` etc., producing valid JSON that `JSON.parse()` accepts.

Additionally, the `partialParse()` call in `MessageStream.{js,mjs}` should be wrapped in a try/catch so a parse failure during streaming does not crash the entire response.

## Reproduction

```js
import { partialParse } from "./_vendor/partial-json-parser/parser.mjs";

// Simulates model outputting PHP namespace backslashes in edit tool JSON
const input = '{"edits": [{"oldText": "use App\\Http\\Middleware\\HandleAppearance;", "newText": ""}]}';
partialParse(input); // throws: Bad escaped character in JSON
```