fix: stop double-JSON-encoding scalar error fields in Box API errors

[buptliuhs]

Summary

When building a BoxApiError for a 4xx/5xx response, the scalar error fields
(code, request_id, help_url, message, error, error_description)
are passed through sdToJson, which JSON-encodes its input. For string
values that adds a layer of quotes: responseInfo.code comes back as
"\"storage_limit_exceeded\"" instead of storage_limit_exceeded, and the
composed error.message inherits the stray quotes.

Reproduction

Box returns this (clean) response body for a storage-limit 403:

{
  "type": "error",
  "status": 403,
  "code": "storage_limit_exceeded",
  "help_url": "http://developers.box.com/docs/#errors",
  "message": "Account storage limit reached",
  "request_id": "<request-id>"
}

The resulting BoxApiError (relevant fields only):

Before

{
  "message": "403 \"storage_limit_exceeded\" \"Account storage limit reached\"; Request ID: \"<request-id>\"",
  "responseInfo": {
    "statusCode": 403,
    "body": {
      "type": "error",
      "status": 403,
      "code": "storage_limit_exceeded",
      "help_url": "http://developers.box.com/docs/#errors",
      "message": "Account storage limit reached",
      "request_id": "<request-id>"
    },
    "code": "\"storage_limit_exceeded\"",
    "requestId": "\"<request-id>\"",
    "helpUrl": "\"http://developers.box.com/docs/#errors\""
  }
}

After

{
  "message": "403 storage_limit_exceeded Account storage limit reached; Request ID: <request-id>",
  "responseInfo": {
    "statusCode": 403,
    "body": {
      "type": "error",
      "status": 403,
      "code": "storage_limit_exceeded",
      "help_url": "http://developers.box.com/docs/#errors",
      "message": "Account storage limit reached",
      "request_id": "<request-id>"
    },
    "code": "storage_limit_exceeded",
    "requestId": "<request-id>",
    "helpUrl": "http://developers.box.com/docs/#errors"
  }
}

Note how responseInfo.code carries embedded quotes ("\"storage_limit_exceeded\"")
while responseInfo.body.code — the raw parsed payload — is already correct.
The fix makes the two agree.

Root cause

src/networking/boxNetworkClient.ts extracts these fields with
sdToJson(fetchResponse.data['code']). sdToJson serializes to a JSON
string, so a string value gains surrounding quotes.

Fix

Read each scalar field with an sdIsString guard, which yields the raw
string and is safe on absent keys (unlike getSdValueByKey, which throws on
a missing key). context_info (a map) is untouched.

Tests

The repository's tests are the generated integration suites under src/test/,
so I've left those untouched rather than add an out-of-pattern unit test under
src/networking/.

Verified locally with a throwaway test that mocks nodeFetch and drives a 403
through BoxNetworkClient.fetch: it fails against the current implementation
(responseInfo.code === "\"storage_limit_exceeded\"") and passes after this
change (responseInfo.code === storage_limit_exceeded), with error.message
losing its stray quotes. Happy to contribute that as a proper test if you can
point me at where error-path coverage belongs in the generated layout.

Breaking change

No type signatures change, so consumer code continues to compile. But the
runtime values of responseInfo.code, responseInfo.requestId,
responseInfo.helpUrl, and error.message change — they lose the erroneous
quotes. Anyone who worked around the previous quoting will need to adjust;
anyone doing the natural code === 'storage_limit_exceeded' comparison starts
working correctly. Additionally, a present-but-non-string field now yields
undefined rather than a JSON-encoded string (Box's error schema always
sends strings for these). Suggest an appropriate version bump / changelog
note.

Note for maintainers

Same codegen question as the companion export PR (#1558) — if
boxNetworkClient.ts is generated, point me at the generator source and I'll
move it there.

[CLAassistant]

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